Netrunner - Card disappears from my oppenent's view when I flip a card over

During our games, I (playing “Corporation”) place cards face down on the table in “data forts”, and during play they become activated when the Corporation “rezzes” (flips over to activate their capability) the card. When I do so, my opponent (playing “Runner”) sees my card disappear from the playing surface rather than being flipped over and exposed.

What might be happening? Why does this seem to happen when I am playing the Corporation, but when my friend is playing Corporation, we don’t experience this issue?

Check the settings on the Mask trait for the card in the module, that must be what’s implicated here.