Why cant I see mapped network drives when I attempt to open a document in Word 2016 for only one user?

I have one workstation in one organization experiencing an issue with opening documents on a mapped network drive. The user opens word and then selects File > Open to view the mapped network drives. However, the mapped network drives are not listed. I can save locally to the PC, I can save to any device under Network locations but the mapped network drives are not there. I opened Excel, Office and PowerPoint and attempted the same File > Open and I have access to the mapped drives in those applications so it appears to only affect Word. I logged into the workstation with another user profile and replicated the issue in Word so it doesn't appear to be a user profile issue. About 15 minutes after I investigated the issue, the user reported that she rebooted the workstation and now has access to the mapped drives in Word when she selects File > Open. The issue appears to be intermittent and fixed (sometimes) by a reboot.
The user is working on a new Windows 10 machine that was deployed about a week ago. The user said she did not have the issue with her old Windows 7 machine.
This sounds like a possible permissions issue but I don't understand why if it's a permissions issue if it is only happening intermittently?

Added info from the user: Yesterday the issue happened the first time after a 30 minute period of inactivity but I had my Word document left open when I was away from the computer.  Today it happened fairly soon after I opened the document I was working on and I had not locked or walked away from my machine.  I will often open my word documents directly from the Explorer window/network drive location rather than File/Open in Word – then when trying to Save the document back to the network I get this message (again, intermittently).  Not sure if that helps but thought I’d mention it.