I'm also seeing this issue and this is the only thread I've found about it. I'm thinking that maybe a reinstall of the TestFlight app might fix the issue, but I'm not sure what will happen to all of the betas I'm signed up for if I try that. As I understand it, the betas I'm signed up for are tied to my Apple ID, so they should just reappear in the app after a reinstall, but I'd like to know that for sure before reinstalling.