izzywave,
Thanks for following up with us on this issue!
If this is occurring on a Mac then it is possible it is from Optimize Storage being used: How to free up storage space on your Mac. That feature will remove files automatically when storage space is needed, so it could be one cause if you use that feature frequently.
If this issue is occurring on an iPhone, take a look at the guidance for accessing files offline with Files app: Use the Files app on your iPhone, iPad, or iPod touch
Scroll down to the section titled "Save a copy of your file locally".
Have a great rest of your day!