I've found a way to "fix" this sync bug.
I checked Sync Internals > Sync Node Browser > Device Info to find out the version of Opera on my old PC. Then I downloaded the same version, which was 80.0.4170.16 (I stopped using Opera in 2021 and came back this week).
The bookmarks synced correctly in that version, so I think they need to be updated to work in a newer version. When I change a folder or a bookmark name, it syncs with the newer version, but changing the folder name only shows the folder and not its content.
I also noticed in the Sync Internals page that editing a folder or a bookmark in an older version of the browser would show as a new entry in the
Type Info >
Total Entries of the newer version.
I took some screenshots to show how to update an old bookmark:
Opera 98: Sync Internal before editing a bookmark in v80.
Opera 98: Sync Internal after editing a bookmark in v80.
Opera 98: Bookmark folder appears after adding a space in v80.
Opera 80: Same folder. Adding a space in the bookmark.
Opera 98: The same folder after.