Highlighting a pdf in icloud files is saved only some of the times even when exiting via the highlight tool

When I markup files or PDFs in the files app on my iPad, using iCloud Drive, sometimes the highlights save and sometimes they do not. When I exit through the highlight tool icon (to turn off highlighting), sometimes it will say saved to file. Other times I wait for it to save, but it never saves. I have reopened the highlight tool and closed it again and it still does not save. It seems to work the first few times then stops. Is there a way to force it to save?