Following up my post of 16 November:
I'm running Sonoma 14.1 on an M1 MacBook Air.
A discovery! When I backup to one of my USB drives, get the Backup Delayed message, the first line beneath the TimeMachine icon still says, as before, that the last backup was (a) to a different drive and (b) on 22 October.
BUT when I go down the menu and choose OPEN TIME MACHINE SETTINGS.., one of the icons has beside it the name of the drive I've just backed up to, and the correct date and time the backup was completed.
So I won't worry any more. (Though I wish Apple'd fix this minor bug.)
If and when I buy a new drive (or reformat this one), I'll be interested to see whether TM's first-line message is still wrong, but it isn't a big deal.