Files app: empty grey boxes where handwritten notes should be

Hi, I've seen similar posts to this but not the exact same thing. On my iPad files app I take notes using my Apple Pencil. I am always sure to properly close out so I don't lose my work, however on some files (not all), there is an empty grey box around where my handwritten notes should be. The notes still appear in the previews on the left-hand side (where you can see each page vertically) but nothing is there when I pull up the page. I have plenty of memory available. This is really frustrating to lose my work.