Maybe a localization issue, as I'm a German user.
Anyway, while I can upload new builds to TestFlight and distribute them just fine, I can't edit the test information in the build's detail view!
The text field simply is not editable.
Is that related to my role? I'm developer at the account.
Any if yes: a) why isn't that displayed? b) that doesn't make any sense. If I'm allowed to upload and distribute the binary to our testers, where is the harm to edit the test readme.
To answer my own question:
https://developer.apple.com/support/roles/
So, indeed developers have only read access to builds, and can not add the test information when uploading a build.
So, it's writing an email to an App Manager each time, who has a lot to do and has to copy the information from the devs, which usually happens way after most testers have already installed the build.
This is not a good workflow, to put it mildly.