Comment 18 for bug 1321354

For the question why the very old release didn't show the problem I'd assume they held something as an open file. This is usually not needed and often fixed after some time, but a behaviour like e.g. keeping the root of the mount open would explain why it wasn't breaking there as the windows definition says:

"... An idle connection is defined as a connection which has no existing open handles (no open files, directories, search contexts, etc.), and no pending operation. ..."

So other than configuring windows to not drop this (as outlined in comment #7) I wonder what one can do.
I can't speak for the apps that use it like Nautilus that was reported, but a "normal" local share shouldn't care much. The Apps might fall into a loop of try-to-refresh-fail-..., but if just mounted there should not be anything.

One of the last reports mentioned ongoing traffic, it might be worth to use iptraf to check what connection that is. Is it to Samba, if so use wireshark to track down what. If the former assumptions on the auto-disconnect are true then we might see reconnect attempts or something like that.

[1] holds some background on various timeouts in windows cifs shares. [2] confirms this applies at least up to windows 7.

[1]: https://blogs.msdn.microsoft.com/openspecification/2013/03/19/cifs-and-smb-timeouts-in-windows/
[2]: https://support.microsoft.com/en-us/help/297684/mapped-drive-connection-to-network-share-may-be-lost