Flag change for coalition not possible

I've tried it over several weeks in more then 4 games and it doesn't work. I've figured out that you can only set a flag at the founding of the coalition, but if you want to change it you can only change it to the standard flags or if you start up a new coalition. I don't know if it's just a bug for iOS or if it's an general a bug for Windows and iOS.

pls fix this