I had this very same issue. Files I opened from emails were saved in data/user/0 which is a directory you cannot edit. The thing that fixed it was to copy the files I needed to other locations and then reset the app. After that the files were gone.
How do I locate data/user/0? What app are you saying to reset? Sorry, I just don't know how to do that, but yours has been the first possible solution that seems to make sense. Your help would be appreciated. Thank you.
This applies to files that were opened in Word. You cannot see data/user/0, this is a hidden file that you can only see if you have rooted your phone; some of the files in your "recents" list are placed in there by the app. I do not know why they did this; it makes little sense that you would be unable to delete files that you only needed one time. After doing this I freed up several gigs of storage on my phone.
If you go into settings in the Android app there is a button marked "Reset Office" . This will delete all the files, so make sure you have saved the ones you need in another directory. When you go back into Word the files will be gone. Attached is an image of the settings which shows the reset button
Thank you. I will try to follow your instructions.