Yesterday evening I tested this on my home network, using my Home Workgroup, saving from one computer, to another computer in the same workgroup. No Domain Controller, no servers, only two computers on the same internal network. The exact same result occurred
of the files not saving.
The issue isn't saving to Mapped Drives, the issue is saving to a Network Location. Any Network Location. It doesn't matter if it is Windows internal sharing, a NAS, a Mapped Drive, a SAN or a Server. All that matters is that it is a folder location not
physically tied to the device doing the saving.
As Rene mentioned in one of his posts, previous versions of Office 2016 did not have this issue, it began occurring sometime recently, I did not have a user using this functionality on Office 2016 until 2 weeks ago, so I can not confirm that.
As in my case, I have tested this across devices using various version of Windows, including server versions, fresh installations, etc... it doesn't matter. I've been providing IT support for nearly 20 years, I have tracked this issue to what I can identify
as a single point of failure, and that is Outlook 2016.
As for using OneDrive, that would be a great solution, if I had my 500+ users all trained on the nuances of OneDrive, but I don't, that isn't planned for rollout until Q2 of next year until AFTER we get everyone onto Windows 10 and are able to use online
only syncing to reduce the amount of storage used. I can't very well equipped all my users computers with 4TB harddrives to sync everything they may need locally.
In the meantime, this is not an isolated or unique incident, it just so happens that the situation is outside of the general use-case and would only apply to someone using the exact situation of saving multiple attachments simultaneously to a network location
from Outlook 2016.