This just started happening.
It used to be as I like it : alt-tab switches between windows only in the current workspace. Now the switcher window has apps from all spaces.
How to fix? I’ve search the dconf for something that looks like it might be the key but haven’t found anything promising.
Look here:
System Tools > dconf-editor > org > mate > marco > global keybindingsAt issue is the system response to the key. I expect the popup dialog to show icons for windows from only the current workspace but I am seeing icons for windows from all workspaces.
I fixed the problem by going into Compiz, enabling the Application Switcher, and then ... disabling the Application Switcher 
In the process of enabling it Compiz asked if I wanted to disable the old conflicting actions (there were two "next" ones and two "previous" ones). I'm pretty sure disabling those is what fixes everything ... but again I don't know exactly what I did, so the only way I can suggest reproducing it is to enable then disable Application Switcher in Compiz.