Time Machine backups disappearing

Apple Support doesn't understand Time Machine.

The answer is simple. Backups become "thinned" (deleted) on schedule. TM retains hourly, daily, and weekly backups to the extent possible within the backup drive's capacity. Hourly backups are deleted after 24 hours except for the first one of the day, which then becomes the "daily" backup. Daily backups are deleted after a week, except for the first one of the week, which then becomes the "weekly" backup. Weekly backups are retained until TM needs the space they occupy, after which the oldest "weekly" becomes deleted.

This assumes you did not constrain TM to run less frequently than it will by default but if choose to do that older backups still become deleted according to that schedule.

Summary: They are not disappearing at random, Time Machine is running properly, and Apple should have known better than to posit that theory.

Apple really ought to read this site more often. They might learn something.

I might add... don't read too much into the Finder's representation of TM backups. In this specific case those dates are probably accurate, but if you delve too much into the Finder some of what you see will not make any sense. In the past it was very possible to corrupt backups by using the Finder to alter files (deleting them, etc) but a recent macOS release rendered TM backup drives "read only".

We could always use the Finder to benignly observe a TM backup drive's contents, but I surmise the temptation to alter them was irresistible. So we can't do that any more. We probably ought not to even be able to open a backup drive in the Finder, but such a change would only feed speculation of Apple of surreptitiously hiding something. The amount of misinformation regarding TM is bad enough as it is.

it points toward a software-related glitch in how Time Machine is managing and indexing backups, possibly introduced in recent macOS versions like Sonoma, or even Sequoia. Time Machine creates local snapshots and manages space automatically, but it should not randomly delete backups without any storage pressure. A few things you could try are resetting Time Machine settings, reformatting the drive (even though APFS is now recommended, testing with Mac OS Extended (Journaled) might help), or using Terminal commands like tmutil listbackups to check if the backups are truly deleted or just hidden.

Setting up a fresh backup on a newly erased drive could also help rule out any configuration issues. If the problem continues, it’s a good idea to report it through Apple’s Feedback Assistant.