Recurring Safari history disappearance on macOS and iOS with iCloud sync

Hi.

I use Safari on macOS and iOS. I have iCloud sync enabled on both devices.

A few days ago, my entire Safari history disappeared and then started saving visited websites again. The default history retention is set to 1 year, if that matters. The whole thing was strange because, despite the supposedly empty browsing history, the History.db file was still 85MB (meaning there were probably some records inside).

A few days later (I was away on vacation), I restored History.db from a TimeMachine backup. I lost a few days of browsing history, but that's okay.

The next morning, the same thing happened again. I restored the backup again, and the entire history repeated itself. And again. And again.

Through trial and error, I concluded it had something to do with Safari on iOS. I usually use Safari on macOS all day, and everything's fine until I shut down my MacBook. Only in the evening before going to bed do I use Safari on my iPhone. Then it seems that iOS is trying to sync Safari's history with iCloud, something goes wrong, the history doesn't sync, and it starts saving again.

Then, after launching Safari on macOS, it syncs to a state that iOS forced, and all the history disappears on macOS, except for the records resulting from using Safari on iOS the day before. At this point, the History.db file on macOS is still 85MB.

I've tried turning iCloud sync off and on, both on macOS and iOS, but the same thing keeps happening. I have no idea what to do with it anymore, and I'm about to go crazy because I have to restore a TimeMachine backup every day.

What could be going on here?

[Re-Titled by Moderator]

Original Title: Safari history disappeared