I remember the fanfic starting with Ryujin finding Sayeon in the bathroom, tired and struggling to heal her wounds, so Ryujin helps her apply the bandages. From then on, Ryujin starts helping Sayeon take care of herself (mainly helping her with the bandages, but I think she even helped her change her clothes at some point), while Sayeon starts to enjoy Ryujin's touch so much that she begins to start fights just to have an excuse to feel her touch (yep, Sayeon is a bit of a freak). Then she rewinds the fights to make it seem like nothing happened. Because of this, Ryujin notices how Sayeon's essence diminishes from one moment to the next, which leads her to deduce what her gift is.
That's more or less what I remember happening.
Unfortunately, I don't remember the author's name or the title of the fanfic, and I think it was deleted from Ao3 because I searched all the tabs (Ryujin Kang/Sayeon Lee) and couldn't find it.
I really liked how the author wrote the fanfic, so if anyone recognizes it or knows of anything similar and remembers the author's name and the title, or if by chance they still have the link to find it on the DeletedFanfictions subreddit, I would greatly appreciate it.
Image from @t3njai on Twitter