Thats odd, I've got some from 20 years ago and theres nothing wrong with it. I guess is possible its just different kinds. like everything they make similar items 50 different ways.
Another interesting use for conductive black foam "Pressure sensors" get some copper foil and conductive glue, glue 1 piece of foil to each side, as you squeeze the plates close the resistance decreases. you can make a very basic scale, or something. great project for kids.