I have had the same issue,with that one set from the original release, I painted one sprue, and put the figures to one side and forgot about them for a long time,maybe two years or more.When I touched them they broke. The ones in the box were still perfectly soft. So was it the paint,sunlight or a combination ? But EVERYTHING else before or since that set has stood the test of time ....and I have painted a lot of figures from a lot of Hat sets,many thousands in fact,and that one sprue from that one set have been my one and only problem. My own thoughts are that its down to human error and someone got the plastic mix wrong at the factory...once, but I stress that is a guess on my part.