Bookmarks don't save

I am using Adobe Acrobat XI Pro, version 11.0.5, on a PC running Windows. I have been editing a large file without any issues up until a few weeks ago, when it suddenly stopped saving newly created bookmarks. It seems to be a particular issue with saving nested bookmarks - it will save the bookmark if it's at the parent level, but if I create it as a nested bookmark or move it to a nested position, it will be gone after I save and reopen the document.

I have tried to troubleshoot it based on what I've read in other forums, but nothing has worked. I've tried Save As instead of Save, optimizing, moving the Preferences folder, turning it off and on again....!

Any suggestions would be greatly appreciated!