Bear in mind that any airfoil when it stalls does not loose all it's lift. In fact most don't even loose lift but continue to increase lift despite being stalled. What does happen in all cases is that the drag from the stall increases a LOT and it's that drag that slows the plane down so far that it suddenly loose lift due to the loss of airspeed.

But a flat plate may well be one of those that suffers loss of lift when it stalls.