@leocg No it is not the extensions data. It is just a normal Bookmark Folder created by an extension, but opera does not sync with other devices.
The "Imported Bookmark" folder is also created when importing bookmarks from other browsers. But, multiple other forums simply suggest to move the content from "Imported Bookmark" to "Bookmark Bar", which is fair in case of bookmark we import from other browsers. But, in case of extension the bookmarks automatically moves back into "Imported Bookmark" folder. It would be great if opera synced this folder also (similar to how it syncs "Other Bookmark" Folder)