Disappearing files

I am using Adobe Acrobat on my iPad to grade student papers with my Apple pencil. The files are on Dropbox, but set as "available offline." Two issues. First, sometimes when I add comments with my Apple pencil in the margins of a PDF, the file will seem to save the additions but when I go back, the file does not have any comments (i.e., it's the original PDF). In the past when this has happened I have found, weeks later, the edited file in some random other folder on my iPad.

Second, occasionally the thumbnail of a pdf will change to some random image of another file not related to my current work and will then sometimes open the file that I want as the random file visible in the thumbnail. For example, a student paper will show a thumbnail of an Italian journal article and will open as that article even though the article is in some other folder.

I have been losing a lot of work and having to regrade so many papers! Any help would be appreciated.

VM