Encrypted APFS drive not unlocking on one of my macbook pros

Hello,

I have just upgrade to Big Sur on my two macbook pros, and reformatted my two TimeMachine backup drives to APFS volumes (each external HDD has two volumes, one normal and one encrypted) that I want to rotate between two Macbook Pros on a regular basis for backups. I want the backups to be on the encrypted APFS volumes.

Both external HDDs work just fine on one of my Macbook Pros (the one I formatted the drives it turns out), but neither encrypted volumes of the external HHDs can be unlocked on my second newer Macbook Pro; both normal volumes do mount ordinarily.

I tired restarting the newer macbook, reformatting the drives, using different keyboards for the input, copy/pasted the correct passphrase, and even tried DiskUtil GUI and commandline to unlock the volume only to get no result.

Any suggestions?

A