Hi,
I"m so glad I'm not alone. We are having this issue too. Files vanish sometimes when user Saves. Additionally, Office's supposedly hidden temp folder/files are visible. User sees them appear then vanish.

Occasionally, users will save files and they will seem to vanish. They turn invisible but keep their names. When a user calls to tell me his file disappeared, I have him press shift-cmd-period to show hidden files. If he reports that it's there but "gray"
then I know what to do.
The current workaround is to open a new email in outlook, then drag the "hidden" file into it as an attachment. Then drag the file back out of outlook to save the attachment. The final file has [1] appended to it if saved in the same location as the original.
I have the user then delete the original and rename the new, removing the "[1]".
Light years from ideal with an office full of people trying to get work done and meet deadlines, but it is what it is I suppose until it's fixed.
Clients are MacOS 10.13 or 10.14, latest minor release on all. Server is MacOS 10.12 (RAID 1+0).
Office version is 16.31.
Thanks,
Jeff