Could it perhaps make a difference if you have Windows 'mapping' the drives on your NAS?
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So perhaps in your case SMB protocols are also a factor?
I would be hugely surprised if there was any low-level difference between (1) accessing files under "\\fileserver\music" and (2) mapping "\\fileserver\music" to "M:\" and accessing files under "M:\". I'm under the impression that mapping network drives is really a Windows Explorer artifact that provides a shortcut reference to the same logical filesystem entity.
The whole question of preferred SMB protocols between the two sides is why the settings were changed. As I understand the discussion on the Synology forum, the revised settings forced both sides up to SMB3...or something else, as the settings also seemed to give preferential treatment to SMB2 leases. The whole thing seemed a bit hand-wavey, since some of the fixes seemed to work for one person's problem and not for others.
My last experiment before I give up and live with the anomaly is to change the nightly NAS backup to every other night and see if that matters. (The backup runs on the NAS itself, backing up directly to an NAS-attached USB drive, so any Win10/SMB protocol stuff shouldn't be a factor.)
And thank you for actually thinking about the problem itself instead of just busting my chops about which version of MB I'm running.