Responses
If you read the comments, the issues people have are not the mere fact that he lost weight. The issue is the overall pattern within manga (and many media outlets in Asian countries) that to have a happy ending you MUST ALSO lose weight. It is a common enough trope that when it happens again in a manga people like it becomes frustrating.

THE COMMENTS BEGGING HIM TO STAY CHUBBY IS TAKING ME TF OUT LMFAOOO IT LOWKEY MAKES EM SOUND LIKE THEY HAVE A WEIGHT FETISH I mean, the uke himself says eating was the most important thing in his life before, now his boyfriend is. Meaning he used eating as like a dopamine source and maybe as a coping mechanism? When he gets sad or anxious, we see him eat more and it kinda hints at that!
So him loosing weight isnt really about the author wanting him to "look better" or anything. Its showing us that the uke has grown as a person and now has a healthier way of living. I say this as a chubby person who used eating as a coping mechanism before!! Recently, i lost 5 kgs because my life has been going a better way, less worries and less anxious. I dont hate my body, but i hate my eating habits.