This is an issue which seems to have been introduced in one of the recent updates - probably the 1.11.1 (23 March) patch to the March release, given that I haven’t noticed it before.
With the time to a Meetup being close but more than 5 minutes (around 10 minutes to go, say), the Join Meetup button briefly appears for approximately 1 second, then disappears. This repeats every 15 seconds, appearing and disappearing right up to the 5 minute to go mark, at which point (as before this update) the Join Meetup button appears and so does the pop-up window with an auto-select to join the button.
I’ve had this happen on four Meetups out of four so far.
This is on Apple TV. I don’t know if it happens on other platforms.