Annotations sometimes not saving on PDFs

Hi there,

I download a PDF for a class from online and save it to a folder in the "Files" app, which is "On my Ipad".

Next, I open it and mark it up using my Apple Pencil.

Next, I click "Done" to save the markups

However, sometimes my markups never save and I lose all my markups

A few of my friends at Carnegie Mellon have also faced a similar issue.

Please advise