Hmm. I wonder if the option on the watch isn't just telling the phone to download it.
. (the radios on the watch are quite weak - not the best performance downloading to the watch unless it's via your phone in close proximity).
I just kicked off the WatchOS Beta 2 download on my iPhone - same - downloading to the phone first.
You should be using feedback to report the battery drain (the more info the Apple devs get, the more likely they are to find the race conditions causing these). Mine are still around 2%, with 3 of them in use.
You could try re-pairing the airtag to see if that doesn't wake up the software update in Find My that will need to be pushed over. (the update will be sent to the airtag over bluetooth).