The suggested workaround didn't even work for us. We've had builds that had expired, and we still couldn't add new testers into it. So I don't even recommend to try their "workaround" as it is waste of time.

I sent feedback 4 months ago. I got one reply but that didn't help at all, and I've sent multiple follow-up emails with no response from Apple. I bet this isn't going to get fixed anytime soon.

Yeah, I've been waiting them to fix this for the past 3 months.. I've tried all suggested workarounds and Apple's support doesn't give any help either. Seems like there's over 2k following this thread so I assume many people are experiencing this issue. Only solution for me is to publish our test builds to external testing.