In the early days of plastic plaques, the material used would absorb moisture from the air and warp. I believe the material was cellulose nitrate. In later years, other plastic compounds became available and were less susceptible to warping. I believe one was cellulose acetate. For comparison, look at the early Sands Las Vegas plaques which often warped, with the Sahara plaques which didn't have the same problems. B et G made both types. Those that would eventually warp would usually give off an odor of formaldehyde (CH2O, embalming fluid).

Here is a link to a site that has deterioration characteristics for several types of plastic.

https://mmics.files.wordpress.com/2008/04/problem-plastics-check-list.pdf

Note that early dice exhibited some of the same problems as they aged. Also, photographic film and casino dice, particularly if they were nitrate based, would change with time. The nitrate based material was also flammable (nitrates were the stuff of commercial and military explosives).