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.

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.