Somewhat skinny POIS'er here. Male, 187 cm, 70 kg.
Before I started paying attention to my diet, I would always look/feel slightly chubby, even while exercising 4 times per week. I dropped gluten about 10 years ago, after which my face looked much less puffy, and it was easier to maintain a healthy weight. Then at some point 6-12 months ago I switched to an MCAS-friendly diet and my weight dropped super quickly from 80 kg to 70 kg. I was a bit worried at first, but my gut feels a lot better, and I look and feel healthy, so I don't worry about it anymore.
EDIT: To anyone experimenting with diets: Please do the research, talk to a professional if you can, and make sure you get all the nutrition you need!