I know similar questions have been asked before but I just can't believe this can't be solved, so I am going to ask again with specific details.

I have two partitions that are bootable on the internal drive of my Mac running macOS Catalina 10.15.7. Both are encrypted APFS volumes.

Whenever I log in to one of them, I am getting a promt, asking me for the password of the other. I do not want to store the password in the keychain to work around the prompt (for security reasons), in fact I do not want the other volume to be automatically mounted at all. All I want is to be able to choose the boot volume on startup (via the [Opt] key).

I tried

1. adding this line to my /etc/fstab using vifs:

UUID=<UUID of the volume> none apfs rw,noauto

but this does not prevent the prompt from appearing.

2. changing the volume role via

diskutil ap changeVolumeRole <diskId> D

but this returns the error: Error setting APFS Volume role: Unable to set the APFS Volume Role (-69599)

Is there seriously no way of preventing disks from being automounted? Can the Finder be at leat taught to not ask for the password?