Time machine says not enough space - but shouldn't it just delete old versions?

I have two TM disks that I swap over every so often.

I'm using an M1 mac mini, on Monterey 12.5

Yesterday, I did the swap, and the backup to the newly connected one seemed to work ok (it has a backup listed at a certain time last night).

However, it is now telling me:

"Time Machine could not back up the disk “Untitled” because it is nearly full.

Delete some files on this disk and try again."

I thought that what is supposed to happen is that TM automatically starts deleting old backups when the disk starts to get full.

Additionally, the TM preferences box shows this disk (TM_B in the attached screenshot below) as having more free space than the other one (TM_A in the attached screenshot) does. That other one, has been happily doing backups for the past few weeks/months.

Any suggestions for what to do about this?