SPOILERS:
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The first side story and the current side stories are Sungyeons past lives(?). We can see that the two of them both centre around sungyeons pov.
First one being that Hosik died due to illness, and sungyeon being all alone. In the end on that story, we get to see Sungyeon waking up from a nap, showing he has some memories of that life.
The currently new chapter,Special.10, we see a new POV from one on Sungyeon past lives(?), probably set aroung late 1800-1900 due to the enviroment. We see in this life, that this version of Sungyeon rember very clear of his first life( the one where hosik dies due to illness). He is again a master and is still searching for Hosik.
However, I feel like the modern Sungyeon, the one that the story is set in with the babies, Sungyeon might have forgotten about his ast lives, due to the fact that it was never mentioned in any of his flashbacks or POV, but their is such a high chance that he remebers and that why he is so possesive of Hosik.
Hope this helps! Im not entirely sure, but this is what i think
huh im sorry but im lost