We have a situation where a group of users will sometimes open Word documents on a 2012R2 file server and edit files, then save and nothing will happen. They also seem to temporarily lose the ability to browse the computer and have to use the "This PC" link when saving to navigate to where they want to save to.
The only workaround is to save as doc or pdf, close Word, reopen the newly saved doc or PDF, then re-save doc as docx (losing some formatting) or copy/paste from PDF. Otherwise closing Word on its own will lose changes.
They are using Windows 10 - 64 bit - 1708 or 1803, Office 365 (32 or 64 bit) updated to the latest version.
The other commonality is the files originate on the same server and file share. But I use that same server (another file share though) and no issue.