As others have said, I just think it's him dieting and it showing on his face. Usually he can't maintain being this skinny, so his cheeks will probably be fuller at some point again. Personally, I think his face fluctuations are really apparent because he also uses fillers from time to time and that changes a bit of his face's harmony.
A couple of examples of other times he lost a lot of weight:
With Jungkook, I always suspected he did something to chin or jaw, at least some fillers. Before he put his nose implant, I think he used to get fillers in his nose bridge since it was constantly changing. So if I had to guess, his face started to look different from before because his last nose job made the other procedures stand out as well.
I don't know if it helps, but the first time I noticed change in his chin/jaw area was in 2019 MMA: