How do I prevent truncation of my Mac browser history and sync history between devices?

Recently something strange happened with my Safari browsing history. Suddenly one morning com.Apple.Safari.History started burning CPU and a zero-byte ClearHistoryInProgress file appeared in ~/Library/Safari. Before this happened, I had history * through 2020-01-12 on my MacBook, and * through 2021-04-15 on my iPhone. After this happened, I now have history * through 2021-04-15 on my MacBook, and * through 2021-04-15 on my iPhone. though these two histories aren’t the same: Each appears to contain only local browsing, with no (or incomplete) syncing between devices, though in some cases I think recent items from the iPhone are synced to the MacBook, while none are synced from the MacBook to the iPhone. I don’t know why the history on the iPhone ends on 2021-04-15, but it shouldn’t (I didn’t erase history there). In an effort to recover from this I have repeatedly tried restoring a backup of my History.db through 2020-01-12 on my MacBook (after deleting all …History… files from ~/Library/Safari), but eventually (some time after browsing to a new site on my iPhone?) the process repeats. I have also tried toggling Safari use of iCloud on both devices, even deleting all iCloud info on the iPhone when I turn iCloud off there, but this has no effect. How do I prevent my MacBook browser history from being truncated in this way? Better: how do I get the histories on both devices to sync fully through 2020-01-12 (or whatever the history file size limit dictates )? --- If I can get this solved, I'd then like to recover the history that has been lost from various backups of days where this occurred, but that's really a separate follow-on question . --- UPDATE: A month later and the history on my iPhone only goes back to May 22 (a month later than reported above), while the Mac history has shortened a bit more. I understand why the Mac history is shortening (size constraints) but why is the iOS history shortening? Could it be that iOS has a smaller size limit on history, and essentially *imposes* that smaller (shorter) history on macOS if Safari history is synced in iCloud? --- iOS 14.4.2; macOS 11.2.3; Safari 14.0.3