Same issue here. I've experienced this on and off since 2023 (Just checked this in my Apple Developer Program Support email history).
If only ASC allowed one to manually add or remove the internal "App Store Connect Users" TestFlight group then it wouldn't a be a problem at all when ASC fails to do this automatically.
This is extremely frustrating.
Topic:
App Store Distribution & Marketing
SubTopic:
TestFlight
Tags: