Yesterday, half of my testers showed status of "-" and had lost the count of sessions.
Now, that External Testers group has completely disappeared. In fact there is no longer an "External Testers" section in the left column of the screen. However I do have "All Testers" under "General Information" where I can see everyone, with the correct status and number of sessions etc.
Possibly related: initially I didn't have "External Testers". A random stack-overflow post told me that "External Testers" only appears once you have created an Internal Testers group. So I created an empty internal testers group, then created an external testers group, then removed the internal group. I reported this as bug FB12769342, but Apple say it is the intended behaviour (*). That was all about three weeks ago. It was OK until today; now I can't see "External Testers" again. Maybe I shouldn't have deleted the empty internal testers group.
Do others whose status is showing "-" find that the correct info is shown in "All Testers"?
(*) I'm a one-person business. The idea of having "internal beta testers" using TestFlight before moving on to an external beta test may seem sensible if you're a business the size of Apple, but I'm sure it's not how most of us work!