Ok, the alerts seem to originate from iOS believing this beta build has expired (source: https://appleinsider.com/articles/18/08/31/how-to-stop-the-constant-update-prompt-in-the-ios-12-beta)
But how are those betas built, that this happens? Is there someone manually entering dates? I'd expect it to be a build server that sets the expiration date to now + 4 weeks or something.
Can't be the signing either, otherwise it wouldn't run, would it?