I have the same problem. Signing and releasing via Xcode works, but Xcode cloud seems to not sign the app and its libraries correctly.

According to this Reddit post, it might be related to having a non-ASCII character in your account name (which is the case for me (I'm using a German "Umlaut")).

I've contacted Apple Developer support on this and will update this answer as soon as I get an answer/a workaround