Hello, I know this post is old but we have a similar problem at our organization. We have windows 8.1 with Bitdefender as our anti-virus. Same details, excel locks up the actual xlsx file disappears and is replaced with a 2 files that appear to be a before
and after snapshot of the original xlsx file. One of the files is a .tmp file and the other has no extension. Both files share the same name prefix which looks to be some GUID or Unique id for temporary file naming.
This is a work environment with a slow cautious roll-out of bitdefender, giving Windows defender to users who we have setup on Bitdefender as of yet. The people without Bitdefender never experience this problem; but my boss and I have both experienced the
problem. I removed Bitdefender from my bosses computer and he has not experienced the issue since. I still have Bit defender and noticed that it happens intermittently and is not consistent. Also it does not appear to happen when working on a network drive.
So while I don't have a clear answer as to why this happens, I can say that it appears to be a problem with Bitdefender.