The front defrost is a selection, not a switch like rear defrost so pressing it again will have no effect. You need to press Auto or another vent selection to deselect front defrost.
The last few cars I had with auto climate control did the same thing. I've always just press Auto when the window is defogged/defrosted for a long as I can remember.
I run my system in manual mode (I don't press AUTO.) With front defrost on, if I hit the off button in the middle of the fan control then rotate the fan control to turn the system back on, defrost stays off.