I think it is just the softer plastics that are deteriorating relatively fast, and I'm not sure if it is because they are in sealed bags... as the treads on my TF2 Bludgeon (Voyager tank) have already started melting into the hard plastic they are resting against, but have always been out on the shelf.
The original topic is here on Allspark (a photo is at post #35), which has more examples on the TFW topic.

I remember people saying there was something similar with the rubber pretender shells (the combiner Pretender Monsters), and some rubber tyres... either the soft plastic is melting like an icecube without doing too much damage to other things, or staying solid and melting the hard plastics it is in contact with.
Yesterday when digging out my Pretenders (to remove the inner robots), I found the Pretender Vehicle Gunrunner, which had rubber rings around the wheels for gripping, and they were melted onto the shelf, forming a flat base to the wheel. I'll take a photo later to post up.