The only thing that I could recommend is to export your bookmarks regularly in the future, so you can restore them from there or transfer them to another device.
Sorry, I don't have any better solution by now, but I'd like to support your request because I have had similar problems on various occasions:
when synchronizing, it went the wrong way: it did not update the stored bookmarks with the new ones but removed the new bookmarks on the computer, so the former version was restored on it (although adjusted the "right way" round in Settings).
It happened too many times so I don´t consider synchronizing a safe process on Opera. It might function sometimes but I think it´s better to have your bookmarks exported and stored.
Best posts made by marguerid
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RE: Sync - Missing BookmarksOpera for Windows
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RE: [Compilation]Data Loss After An Update/UpgradeGeneral
There is an extension called Tab Hamster which I strongly recommend. It restores all lost tabs with one click.
Losing all tapes has become sth regular during the last year, but this time it happened two times within 24 h.
Not the only fault. I don`t know what is happening but Opera is getting worse and worse, losing all its former advantages. Been using Opera for decades, so it is hard to leave, but I am looking now for alternatives.