I am a Public Beta Tester. I assume the Big Sur Beta program has completed. Not that Apple actually has announced this fact, but I think it is a safe guess that all development has moved to Monterey.
Apple left me on Mac OS 11.5 (Build 20G71). It simply walked away. It never fed me 11.5.1
So, the workaround is to leave the beta program, which erases the Certificate for the Big Sur Beta.
Then run software update. Then, like magic, the 11.5.1 update appears. Run the update, then if you choose, rejoin the beta program and re-enroll your device. You’ll get a new certificate for Monterey.
Sadly, all communication in the Apple Public Beta program is ONE-WAY. And it does not flow from them to me. Luckily, we have TidBITS.