I wonder: would anybody be willing to try to replicate the following to see if this issue is a general iOS issue?

Step 1: Create a Word document on your PC. Add a bunch of text.

Step 2: WHILE KEEPING THE DOCUMENT OPEN, also open it on iOS app. Modify the doc via iOS.

Step 3: Click the "refresh to see updates" button (bottom right) on PC to see the new changes to the document from your iOS changes. Kill the iOS app. On PC, modify/add text in the document again, then Save.

Step 4: Try opening the document in iOS app again.

This should cause the error, and the document will totally cease to work on iOS.