Time Machine

I have a MB Pro 2019 with 500 GB SSD. I am backing up to an external disk drive with Time Machine.

As far as I understand, when that backup disk drive gets full, it makes space by deleting the oldest backups.

The last time I wanted to backup I got a message that I can't use the disk becaus is full and I must use another disk. Shouldn't it just delete the oldest backup to make space for the new backup?