iCloud not showing latest Whatsapp backup
I'm in a similar boat, having just bought an iPhone 12 pro, and am trying to swap in a SIM from an iPhone 8.
What I've noticed is that the last backup that the iPhone 12 is recognizing was an automatic backup from last month. I think that maybe the new phones aren't recognizing manually requested backups.
I've changed my automatic backup to daily, and am going to try swapping the SIM card in again once a new auto backup has happened. Will update this post once there's a result to report on.
Someone else in a different thread recommended changing the name of your new phone to match the name of your old one, but that didn't work for me. Might be worth a try though.


Update: Switching the automatic backup to daily seems to have done the trick. I put the sim back in my old phone, changed the auto backup, and deleted Whatsapp on my new phone. After a day, I swapped the SIM back to the new phone and then reinstalled whatsapp. The Backup went as expected.
Hope this works for you!