When an input is set to Theater Mode, the display draws an "X" through the Mute button display on-screen to indicate it has no effect, and indeed pressing the front panel Mute button does nothing.
However, if you press "Mute" on the remote, the "Mute" box still goes inverted or back to normal when pressed a second time.
Just a little programming bug but interesting nevertheless (perhaps only to an engineer…)