Dude are you serious!? I dunno, can't believe it based on my experience. I had the same 4 cards for the last 2+ years thru lots of metal detectors way more than a few times a month and never any troubles. And before that the similar cards for many years thru metal detectors etc. And before that... it goes on and on with only one dud that was a newew card (below).
Maybe you are sitting too close to your teevee or your computer or cellphone is too close to the cards?? This is more likely as these items are proven to demagnetize.I dunno it seems like a reach to say it is the metal detectors.
Though I do recall a while back one card was giving me troubles while it was new, (and probably a a placebo effect here) but the clerks would rub the magnetic strip side back and forth on their shirt or pants then rescan. I doubt it does anything but for some reason the card did take when this is done.
I googlesearched 'credit card demagnetize' and found a few hits related to this but they are not substantiated, probably urban legends.
Just call your cc company and get a new card. Maybe this was a bad one in the encoding batch. Good luck.