Can I propose another possibility..... just conjecture.
Perhaps the pen is REALLY magic ink. But that is one that requires a chemical reaction.
Now there are two boxes, one for PAP and one for opposition.
Let's say, the one for PAP is coated with another chemical, that will render the magic ink permanent, so if a voter mark an X at the PAP box, the mark will be permanent.
But there is no special ink coating at the opposition box (likewise, no special ink coating on that piece of paper TS used to write his sweet-heart phone number)...... Because there is absence of special ink coating, whatever marked will disappear after a few hours.
BINGO..... the voters who marked at PAP box will remained a valid vote (the magic ink is neutralized by the special ink coating) but any other place will be vanished.
I become more suspicious because, why the voting slip (ballot paper) is black??? So that one cannot tell the ink coating from one box to another, because the separation is black colour that covers up the unevenness.