Back on topic...
Of all the different things that deteriorate on Transformers toys (rust on MP Megatron's feet, Actionmaster Vehicle rubber going brittle, selective yellowing on some plastics), I didn't know that the rubber shells to the Monster Pretenders
is known to have spots of discolouration, even when left sealed. Either due to mould or discolouration in the added chemicals in the plastic that gives it the different colours... I'm not sure.
(I guess everything is going to degrade eventually... and maybe 30 years is already a good lifespan for plastic products/toys?)