Hi! I use Word and Excel in my Office365 subscription on my iPad. I frequently get the error message at the top saying “can’t upload - we can’t merge your changes with those made by someone else” although I don’t have the file open anywhere else. I thought it might be a Dropbox issue since my files are normally in Dropbox, shared among my iPhone, iPad, and work laptop... but I still got the same error when I tried storing files in OneDrive or in the Files app on my iPad! Yes I’ve reinstalled Word and Excel, powering down my device in between.
I get that message, but only occasionally (not frequently, as you indicate is your experience). My files are all stored in OneDrive. I do go back and forth between a MacBook Air and a desktop iMac, and that use of multiple devices is no doubt a part of what's happening. In fact, it probably IS the underlying cause. I suspect it can be a case where the synchronization with the file-in-the-cloud hasn't happened (for whatever reason that can be).
In the one file that is most frequently involved, a spreadsheet I use to track investments, Excel always give the option to discard changes or save a copy. I choose one or the other, depending on how much I've just done.....and then complete the comparison and update on my own.
I'm not helping resolve your case, I'm sure....just letting you know you're not alone. I don't think the issue is in Word or Excel, however. It's in the syncing (or lack thereof).