Revert to 10.15.x beta updates from 10.16 (macOS 11)

I have a Mac with 3 partitions: Catalina Release, Catalina Beta and Big Sur Beta.

I was running 10.15.6 beta and installed to profile so that I could upgrade to 10.16. I then downloaded the 10.16 installer and installed in on the formerly blank partition for 10.16. Now I want to reset my 10.15.6 system to get 10.15 betas instead of 10.16. I downloaded and installed the profile for 10.15.6 beta 2 but it still wants to update to 10.16 instead of saying that my 10.15.6 beta is the latest.

How can I fix this?