I don't have Crohn's but know several people who have colitis, including a friend who was on sema and had to do a few months on prednisone to get a flareup under control. Their doctor told them to stop taking sema for the duration of the flareup because there could be an interaction between the two drugs, and that sema may be ineffective while on it. I imagine, without even going into what happens with blood sugar, any weight loss would be masked by the amount of water retention people have on pred.
(and before anyone asks why I'm discussing these things with friends lol, they know i'm on a glp1, and they also know my mom and partner (and tbh my dog too) have colitis and sometimes have to go on pred so i know a bit about it)