Hi! Perhaps our experience can help you.
We had the same problem in our organization with members of audiences disappearing after adding them to an app audience and updating the app.
It turned out the disappearing users had permissions on higher level in the workspace itself, which overruled the App audience permissions.
When we removed the permissions in the workspace, the problem was fixed.
The permissions could be added again in the App audience and did not disappear anymore.
I hope it helps someone!
Kind regards, Lil
