The deformed die looks to be Cellulose Acetate. When this plastic degrades it discolors (usualy darkens), and warps as the faces "implode". It will give off a vinegar smell from the acetic acid. The offgasing byproducts of the deterioration will eat the styrafoam and other dice. Heat and Ultraviolet (sunlight) light excelerate the deterioration of acetate (and other) plastic. Kepp your dice away from heat and direct sunlight. Polyurethane and polystyrene foams will eventualy degrade as well and give off nasty byproducts that are damaging to dice.
My dice are curently stored in styrafoam (polystyrene) but this is short term only. I am going to make storage trays with polyethalene foam "ethafoam" which is supposed to be "archive safe" for 20 years pluss.