It's not you.
Same thing here in Xfce using 2 monitors and number of workspaces is 2.
I created a "new file" on workspace 2 and the system put it on the desktop
of my "Primary Display" on both workspaces.

Sorry. Workspaces seems to be working, just not the way you are hoping it would.

Peace.

grudgingly confirmed
 

beware...name of thread could turn out to be self fulfilling prophecy...:doh:

rather than finding a solution to my hypothesis, it appears only to have been proven correct.

the only way to create an empty workspace in workspace switcher is to start with an empty desktop.
what you do on one desktop is replicated in all workspaces, except for panel usage.
[at least in mint 18.1 and Xfce]
not to blame this program, after all it is called workspace switcher.

thanks all.
now...where is the next dead horse grazing, it may already have won a free flogging