You got this fixed? I also had this problem since few months..
I just phoned App Store Supporters and provided them more details. Will update here if we figure out a solution!
I've been having the same issue with 2 different apps and have been working with Apple for over a month. We managed to fix one of the apps but not the other. To fix it, they had me remove the rescue email (which is my personal Apple ID) from my developer Apple ID. It wasn't clear why this should be an issue, but that's what they had me do.
I then had to delete my user from the list of "App Store Connect Users" under TestFlight in App Store Connect and then add it back. Once I did that, I was then able to get nightly builds for that app again.
However, doing the same thing with my other app did nothing. In fact, App Store Connect won't even let me remove my user account from the "App Store Connect Users" list. It either acts like it removes me (but leaves my name there) or gives a generic "something went wrong, please try again" error.
I'm just posting here in case any of that info helps anyone else (and maybe get some additional info).
I'm struggling with this as well. It seems like very new build, testers are magically removed and I have to go thru the somewhat tedious process of adding them again.