It appears that TestFlight has a size limit - our WhatToTest file was well above 2 KByte in size. Once I reduced it to <1K, it again shows up correctly in the TestFlight app.

This issue still persists and is extremely frustrating. We're in contact with european support and have given permission to perform analysis in our account, but nothing's happening. :-( Looks like we'll have to move back to github actions, or look for another CI platform.