For about a month, i was seeing this once or twice a day on my Linux Mint VM and couldn't figure out the cause (and having to umount/mount to correct it). I tried using NFS, various SMB versions, SMB options, all resulted in the same issue. As others have mentioned it didn't seem to be based on the amount of time it was mounted, rather the time after the previous write to that mount.
I never discovered the source of the issue, but I have since switched to using autofs so that it unmounts and remounts the share dynamically. Since going to this, I've only had one instance of this issue come up and that was simply because I was inside the mounted share via command line and autofs couldn't unmount it.
My setup uses a cache drive for writes on one of these shares, but not the other and both experienced the problem. The VM runs off the cache drive in case thats relevant at all. I also run the 'dynamix cache directories' plug in, but have it exclude one of these shares (the one that uses the cache disk for writes). But again, it still happens to both shares.
This has also only occurred on the new 6.8 and 6.8RC releases for me, but I built this server new and went straight to rc1 a few months ago so never attempted to run it on 6.7 and older.