Apple Files Glitching On I-Pad Pro

I am a college student and love to use my apple iCloud files to save my homework files and sheet music. I use the mark up feature to write on these pdf files often. Recently when I write on a file, the entire screen will flash black and restart the file deleted everything I just edited onto that file. This happens more often than not. I have shut down my iPad and restarted it, I have also deleted all apps that are necessary for my studies, as well as updated my iPad to the newest iOS version. Unfortunately, this issue still happens. How can I fix this so that it will stop restarting my files while I am editing them?