G4 "dog tags" discoloring?

[lonewolf]

I've got a bunch of the Enterplay MLP dog tags, and going through them (they were in one big cluster) I noticed on the older ones from 2013, the resin that covers the front appears to be yellowing. The MLP Movie ones I got more recently are still clear. Anyone else here collect these and has had this problem?

[poniesthatsparkle]

I brought out mine recently and the still seem fine so far. I'm hoping this won't be the case for all of them.