Startup Manager does not show new APFS volume

On my 2016 MacBook (running latest Catalina) I went into Disk Utility and added a new APFS volume with which I plan to beta test Big Sur.

I then re-booted, holding down the Option button to get into Startup Manager, but Startup Manager does not show the new volume. It shows only "Macintosh HD".

Went back into Disk Utility, and it shows three volumes: "Big Sur", "Macintosh HD", and "Macintosh HD - Data". It's a 250GB drive, with 121GB free.

Am I doing something wrong here? How do I boot into the new Big Sur volume so I can download the beta?