Excel Missing Saves

We've got a bunch of Windows 10 PCs running Office 365. There's a QNAP TVS-471 NAS being used as a file server. The file server is setup as mapped drive.

Recently several users have complained that some of their work has not been saved despite everything seeming normal at the application end. 

Typically, they'll maybe open up an excel sheet, make some changes and save the work. Only when they come to look at the spreadsheet again, none of the changes were there. It's as though the file wasn't saved. They insist there were no errors or indication that the save failed. The timestamp on the file is the same as the old one so it does look like no save occurred.

I've currently got them to check the modified date after they save to make sure it worked but this is slowing things down and people are getting irate because they're losing hours worth of work each time it happens.

We've been running the NAS file server for a couple of years now and the problem has only been doing this for a month or so. Some people have mentioned new saves 'disappearing' too. One person said a Powerpoint presentation acted the same way but I've had no other report of weirdness with other file types.