Well, you remember how ML gave a antidote before the contract. That was a actually his poison in a very diluted form which should have worked as a antidote in normal case scenario. But due Eui Jae's poision resistance he ended up detoxifying it automatically. So when they made the contract ML's true (concentrate) poision entered MC's body which was more potent that what his ability could detoxify immediately. As such it took a moment before he recovered.
~ this is just my opinion~
As for why ML didn't suspect anything, he most probably has some suspicion but may have thought that something just went wrong or that MC is just weak to his poision.
Oh now that you've explained why the antidote didn't work, I get it now. And yeah seems likely he has some suspicions now but it's not shown yet because we don't have his POV yet.
It's really hard to understand what's going on. Sometimes it feels like the scenes jump from one scene to another without any explanation but it feels like the story is well-written so I guess translation is off sometimes? For example the s+ poison that was injected to MC, why was he poisoned to the point that the antidote was not working? From the way Sa Yeong explained about the contract it seems like he has entered into various contracts with other hunters before, did every one of them got poisoned like this? or was it only Eui Jae who got poisoned badly? If so why? Because if you think about it logically, Eui Jae is a strong hunter and he has detoxification skill as he mentioned so it's strange that he was affected by poison that badly when others before him and who were weaker than him weren't. If this is not the case tho, why was Sa Yeong so shocked by it? he should have expected the effect of his poison especially since he doesn't know Eui Jae's true identity. This scene really did not make sense to me. Because it could've been a pivotal moment where a clue of Eui Jae's strength could've been shown to Sa Yeong. Instead after he recovered they acted like nothing has happened. I had to reread it because it confused me and no explanation was given. Did I miss anything in that scene? And to add to this the last chapter (ch 20) was so confusing ヽ(`Д´)ﾉ