I'm having the same issue now a few months later. I contacted support and am waiting to hear back from them. This is one of many issues I've had with testflight. For being a developer platform the developer experience is subpar. I am trying to get my small business off the ground and I can't even get app access to my designer for them to see the whole flow. If a product manager sees this, know that this is one area of many frustrations that is driving developers to your competitors.

I'm having the same issue right now. It has been working up until this point and without changing anything, suddenly pushing tags it doesn't even find them. I'm trying to build a branch first as the instructions above describe.