I am having this same issue currently, my work around was to create a second "external" group and add my internal testers there. That required me first to expire my builds for my internal testers which was annoying. Related or unrelated but at the same time that was happening, my build that is uploaded to App Store Connect, showing a status of "Testing" and uploaded at 3:45 am this morning, is unavailable in TestFlight on my iPhone (despite my having an invite). Judging by my usage today, my users haven't received the updated build either.