TimeMachine is not removing any old backups.
The disk itself is plenty big enough (1TB, backing up a 500GB main drive), and I have (multiple times) reformatted it to start TimeMachine afresh. After about a month, the TimeMachine disk is full. I last formatted the disk August 19th, and now (September 10th) the disk is full. I can see about 490 backup snapshots, give or take every hour.
The TimeMachine disk is an SSD, formatted Mac OS Extended & Encrypted.
What could be causing this and what else could I try to get TimeMachine to thin out backups as it used to?