Using Adobe Acrobat Pro DC, when splitting by top level bookmarks, all resulting files are the exactly the same instead of different documents for each bookmark. No \ / : * ? " < > | were used in bookmark names. From 9 bookmarks it created 9 new documents and the file names have the names of the various bookmarks, but the image of each of the 9 files is identical to only one of the bookmarks.
That's very strange... Does each bookmark lead to a different page when you click it? Can you share the file?
If you're interested, I've developed a script that does something similar to that function (before it was added to Acrobat). It might work better for you. You can find it here: Custom-made Adobe Scripts: Acrobat -- Extract Chapters by Bookmarks