I'm using Opera 43 on macOS. When I add a bookmark, I am able to select the "Unsorted Bookmarks" folder to save it in. However there is no way to view the bookmark again since there is no Unsorted Bookmarks folder anywhere in the macOS version of Opera.
The "Bookmarks" menu in the macOS app menu shows an unnecessary "My Folders" items which duplicates the contents of the root menu. I believe this "My Folders" menu item should be replaced with "Unsorted Bookmarks". "Unsorted Bookmarks" should also be added to the "Bookmarks" navigation tab.
Edit: It appears that "Unsorted Bookmarks" appears in the "Bookmarks" navigation tab after you have added at least one bookmark to it. However the "My Folders" menu item in the macOS app menu should still be replaced with "Unsorted Bookmarks".
I just looked at the Windows version of Opera. It looks like the contents of "Unsorted Bookmarks" must appear in the root "Bookmarks" macOS app menu, with "My Folders" showing the contents of "My Bookmarks". Right now, the macOS version shows the contents of "My Bookmarks" in the root "Bookmarks" macOS app menu instead of showing the contents of "Unsorted Bookmarks".
They made some changes, but if there were some Bookmarks in "Unsorted Bookmarks" folder, they should be there still.
About the other, may depend about changes made on the structure too.
I've started with a new browser profile. The contents of "Unsorted Bookmarks" no longer show up in the root "Bookmarks" app menu. The app menu is instead incorrectly populated with a copy of "My Bookmarks", which is already displayed under the "My Folders" submenu.
Is there the same content?
Maybe it's not the same folder despite it's name.
The entire "My Bookmarks" including its subfolders is duplicated at the root menu.
Then, I just can tell you to take a look at sync.opera.com to check if there, they are duplicated too, and if need it, use an extension to delete duplicates. (Opera has it's own system too, but haven't tested and I don't know how many time it takes) SuperSorter or Bookmarks checker can do the trick. Download Chrome Extension Needed
There is nothing wrong with the data. Opera for macOS has a bug where it doesn't show Unsorted Bookmarks in the menu. It displays correctly on Windows.
But it's a known bug or because your test you have figured out?
If so, should be tested for other users and if needed be reported (Unsorted Bookmarks are deprecated).
Please fix this. Unsorted bookmarks are missing from the Bookmarks menu on macOS, and there is no way to add new unsorted bookmarks from macOS if the folder does not already exist.
Unsorted is still there. You might have to scroll a bit, though:
Looking at this webpage
Unsorted bookmarks are still there, you may have to scroll all the way up though. Bookmarks - Add Bookmark
Or you can open Bookmarks-Show All Bookmarks and scroll the left hand frame to the top:
(I've edited this about six times. The arrow I was drawing was screwing with the ability to show an image)