Time Machine backups failed after simple Sonoma update...

Mac Sonoma - I had started a new time machine a few weeks ago.

- I believe there was many gigabytes of free space on the external SSD drive.

This morning I updated to the newest Sonoma version.

- then I ran Time Machine back up after completion.

- for some reason the latest backup was not able to complete and the error said that the external SST did not have enough free space (there was still 50 gigs free).

- on the disk was only one back up instead of the 10 or 15 previous days which had been deleted for some reason.

- nothing else had changed on my drive except the macOS Sonoma update.

- it seems there's no reason that it would've used up so many gigabytes after a simple update.

I thought Time Machine was supposed to remove old backups when space was needed?

Q: so I'm curious why this happened and if it has happened to anyone else?

Q: does anyone know any way to recover the existing Time machine back up?

Thanks – Dave