I have a button on the side of my mouse that Mission Control sees as Mouse Button 4.
In Mission Control's preferences, I can assign this button to either 'Show Desktop', 'Mission Control' or 'Show Dashboard'.
All three of these works fine, but if I try to map it to 'Application windows' the mouse button doesn't respond.
I'm able to bring up 'Application windows' using the keyboard shortcuts though.
My new iMac is 2 days old, so it's on fresh and up to date macOS Sierra. Same problem on my brand new MacBook, again with macOS Sierra.
Is the mouse shortcut just broken for this? Can anyone else reproduce it?
[Updated] My mouse is a Logitech G400 Optical Gaming Mouse. I've since installed the Logitech driver software, which also allows you to assign a button to Mission Control (which I did, and then unassigned it from system prefs).
This doesn't work properly either, but rather than not responding, all windows move a couple of pixels and then revert back to their original position. Almost like its trying to invoke Mission Control but something is aborting it. I can map the buttons to any other feature and it works fine