When muffin runs as the window manager (instead of xfwm4), there are only 4 workspaces available, also after switching back to xfwm4. As it seems, muffin reduces or fixes workspaces to a count of four. Normally, I use five workspaces and I can increase them also with running muffin. Xfdesktop or xfwm4 should reset the workspaces count when xfwm4 is active. xfwm4-4.11.0-1.fc20.x86_64 xfdesktop-4.11.6-1.fc20.x86_64 muffin-2.2.3-1.fc20.x86_64 xfce4-panel-4.11.0-1.fc20.x86_64
If muffin changes the workspace count there's really nothing xfwm4 can do as there's no way for xfwm4 to know that muffin was the cause rather than you wanting it that way (this wouldn't really be something for xfdesktop). This is going to have to be something the muffin developers would have to restore when muffin exits (and hope the PC always shuts down safely). Sorry, there's not much we can or should do on this end.
Since muffin was reduced by the developers to be solely and only a lib for cinnamon (as a specialized fork of mutter as a fork of original metacity), not an own window manager any more as its own standalone, this bug gets invalid and unfixable.