Hi,
You have misunderstood the meaning of the ‘Any Player’ option in the Mask trait. That is who can mask the piece, not who can unmask it. The difference between Any Player and Any Side is with ‘Any Player’, only the player who masked (i.e. dealt) the piece can ever unmask it. With the ‘Any Side’ option, then if the player who dealt the piece leaves the game and a new player comes in and takes the same side as the masker, then they can unmask the piece.
You do not want to be using a Mask trait at all. The only function of a Mask trait is stop other players from viewing a masked card, whereas you want everyone to access the cards. You want to be using a Layer trait instead that allows all players access to the card.
Regards.
The ‘Any Player’ option means that