Does the PDF file have any security restrictions?
can you share a link to that pdf so that we can replicate this issue at our end?
No, it doesn't (no restrictions, I have been continuously checking this).
It happens, in fact, to not just one, but to all the files kept on my computer, but I could, if you want, produce a little minimal example and upload it to the document cloud.
I can now also restrict the behavior to only second++ level bookmarks. Producing and saving first level bookmarks seems to work fine. When putting a new bookmark and expanding it to next level, the whole thread (i.e. the first bookmark, as well as its branches) all get lost after saving and reopening.
I have furthermore tried to repeat the same error on a single page test pdf file, but was not successful. It seems therefore, that the error occurs only (?) with big files (scanned books).
Eagerly waiting for your help.