Try looking for Bookmarks.bak file on your profile folder - on every of your Pc's -.
OMenu > About Opera
| And follow the Path for profile.
As a security measure, download Opera Offline installer - Opera.com > Opera for Computers - and install it as an Standalone.
Then, use it to "import" the biggest Bookmarks.bak or each of them if necessary.
Rename from Bookmarks.bak to Bookmarks and put them on the StandAlone profile - inside the install folder itself, look for Data folder -.
Do your needed reordering, deleting or whatever you need to keep it as you like and export them, save the file and import them to your current Sync'd Opera profile.
Sometimes Sync it's a mess, as @bbildman says, so BackUps are needed, just copy your profile folder/s as frequent as you consider.
Check out the signature's link to know about important files.
Windows 10 (x64) | Anniversary Update
Opera Stable · Beta · Developer
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