It hasn’t for over a year. I don’t know if there is shielding between the magnet side and the cards, but I can’t put it in backwards and have it hold.

This is the one I use. I have another one that I got for free from a conference that is aluminum and has a lever to push up the cards, but it’s bulky and has corners. The one I linked is slim enough that it doesn’t bother me, the magnet is strong enough that I’m not concerned about it randomly taking off, and will hold 3 cards.