
I'm rather confident JH, did not cheat. TBH, I foresaw this scenario playing out like this a long time ago. JH is facing a reckoning between his past crush and 1st love. I don't really equate crushes on the same tier as love, because they're more like an obsession, and typically come with no satisfaction in the way that love grants. JH is blasé, but I can tell that he himself doesn't even understand fully what's going on with his emotions. This is all my opinion of course, but my theory is that the realization is starting to dawn on him that his old crush doesn't compare to what he has now. The direction the story is taking ultimately will lead to a happy conclusion IMO.

After rereading this a few more times, I think I get where both sides are coming from. YY is used to DH being either indifferent or flirty, and after what has happened thus far, YY sees DH as having no apparent feelings of affection towards him. This is why he up and left dinner so quickly, he felt like DH was insinuating that he wanted to be rid of him, and that his continued presence must be a burden to DH now.
DH on the other hand, during the first part of this series, has treated this whole thing like it's a game of cat & mouse. None of his flirtiness up to that point had any meaning other than wanting to get a rise out of YY, he wanted the mouse to take the bait so he could remain in control of the situation and turn him away. it's true that DH may have caught feelings early on, but his own perception of himself is what has driven his behavior. It's only recently that DH seems to becoming aware of how he feels, but this is an area he doesn't seem to have any experience in. He treated his last relationship with "seriousness", but I think the author wanted to convey that love is too chaotic & unstructured for seriousness alone to work. Therefore this is likely DH's 1st 'true' love, because he's now acting completely out of character compared to the version we were 1st introduced to.
None the less, while it's frustrating to watch, that's what makes this series so fun to come back to. To see what happens next. I sense we're building up to the confession at this point, I feel like it should be DH who breaks 1st. He needs to break that facade and I think it needs to be from a place of desperation, vs calmly during a quaint little dinner.

I'm truly frustrated with how the reconciliation was handled. They just fuck? And once they talk about it (mostly offscreen) they are just "meh" about it. I find this to be exceptionally bad story telling. As a reader, I don't feel any closure. They just have a shallow relationship.

Tapas had changed a few lines in chapter 50. Korean is high content language and there was things in chapter 50 that would be missed by speakers of low-content languages, such as English unless they understand high-content languages. There was a lot more there than them having sex. You can have the opinion that is bad story telling and some is Tapas’s fault by changing the story. That is up to the reader. But I don’t think with all the content of the story and how they feel about each other supports them having a shallow relationship. They just had misunderstandings. I don’t like that most of their conversation was offscreen but at least they talked about it.
In chapter 40, even though they argued and it seemed they were not listening to each other. Both had to rethinks. Hajin already knew Jaehee did something to break them up and he thought Hangyum saw the note and did not call. The talk with Hanwool and the hour thinking about Hajin at the school helped Hangyum. So somethings they worked out by themselves.
Therefore since they can understand each other and have that small conversation with each other. Chapter 51 actually shows how well they fit together. That little bit of dialogue showed how they match well together and work things out with a simple conversation.

Tapas had changed a few lines in chapter 50. Korean is high content language and there was things in chapter 50 that would be missed by speakers of low-content languages, such as English unless they understand high-content languages. There was a lot more there than them having sex. You can have the opinion that is bad story telling and some is Tapas’s fault by changing the story. That is up to the reader. But I don’t think with all the content of the story and how they feel about each other supports them having a shallow relationship. They just had misunderstandings. I don’t like that most of their conversation was offscreen but at least they talked about it.
In chapter 40, even though they argued and it seemed they were not listening to each other. Both had to rethinks. Hajin already knew Jaehee did something to break them up and he thought Hangyum saw the note and did not call. The talk with Hanwool and the hour thinking about Hajin at the school helped Hangyum. So somethings they worked out by themselves.
Therefore since they can understand each other and have that small conversation with each other. Chapter 51 actually shows how well they fit together. That little bit of dialogue showed how they match well together and work things out with a simple conversation.

My thought still remains that the author had no problem spending several chapters sowing seeds of angst and tension, yet couldn't even spare a full chapter to undo them. Decent story telling typically sees these tensions reach a certain boiling point before reaching a final payoff, where the reader finally witnesses scenes unfold which allows their built up angst to abait.
Even if korean is high content, it still stands that the author relied heavily on implications instead of actually devoting panels the reader needs to witness to feel the full impact of closure. That wasn't granted here, this is by no means the 1st story to do this, and it won't be the last. But as the reader, I do indeed feel blueballed at the end of this. That feeling like a huge opportunity was avoided for nothing.

I agreed to your thoughts, “ I don’t like that most of their conversation was offscreen but at least they talked about it.“. I wished we would had seen that conversation. I never disagree with that but simply comment how well they matched and understood each other with the little bit of communication they had. As I had said on my chapter 53 spoiler, there were many things left open and things they added but never came back too. I hope in extras or side stories it would show it.
Not all stories are not same and maybe this reached it a different way but it is hidden by a few things, like Tapas’s dialogue changes and things lost in translation. Maybe when the fan translations are up it would help a little more. But Koreans would pick up on more with the implications there. That is kind of High context languages are. They tend to speak more indirect and the meaning is hidden. Like if I would say “It is cold isn’t?” You know I am directly saying turn up the heater. Koreans probably felt that closure when others would not. That is probably the same with other stories you have read.

“ In cultures with direct communication style (which tend to correlate with task-oriented cultures), such as U.S. Americans, Australians, Germans, and Anglo Canadians, both literal truthfulness as well as efficiency in communication are highly valued and to some extent are a higher priority than personal or political sensitivities, especially in a business setting. Saying "No" or "I don't know" is considered both honest and respectful of the party, since it does not mislead them or lead to "game-playing." Problems are felt to be solved more rapidly if open and frank discussion is encouraged.
In indirect cultures, on the other hand (Japanese, Chinese, Indians, Saudi Arabians, for example), directly communicating negative information is seen as impolite and crude, even in a business setting. In these situations, polite excuses or evasions, which both parties usually know and recognize as such, are given, and in extreme cases even outright fictions are invented—again with recognition by both parties that a diplomatic strategy is being employed. Problems are felt to be solved more productively if they are handled with tact and discretion.”
https://www.watershedassociates.com/learning-center-item/direct-communication-vs-indirect-communication.html

I am just trying to help you. Maybe understanding this will change how you see these stories and get you the closure you feel is missing.
https://www.google.com/search?q=Indirect+Communication&ie=UTF-8&oe=UTF-8&hl=en-us&client=safari#
Oh how the turns have tabled.