If you have time, read this article I wrote on bullying victims. https://www.backyardchickens.com/articles/chicken-bully-chicken-victim-a-two-sided-issue.73923/ It will help you understand how this dynamic works in a flock and how self defeating it is for a victim of flock disapproval. The...
There is the possibility that she's behaving this way with seemingly poor appetite because she hasn't regained her self confidence in asserting herself at the feeder and competing for treats since being returned to the flock. This is especially something to suspect since she's likely still...
Is she currently molting? It sounds like she's doing well except for appetite and weight. Those are two very common side effects of molt. Check her for emerging pin feathers to see if she's molting.
I would try giving her a little bit of animal protein each day. Canned mackerel or tuna, liver...