We received a report (T254950#6605907) of a user on beta with a confirmed email address being unable to create a client using Special:AppManagement. The "Create client" button appears for them, but they receive a email_not_confirmed error when attempting to create a client. I think the best approach here is to try to reproduce this issue in production since this could be an issue specific to email confirmation on beta.
Another user reported receiving CORS errors when attempting to create a client: T254950#6611487. I wasn't able to reproduce this error myself, so we should try with a few others to see if we can reproduce.