Recently, someone on Reddit posted that they noticed a crack on a joint on their MPM4. The part in question is the pin hinge that holds the side window and top part of the door that forms part of the chest in robot mode. I checked mine upon reading the post, and sure enough, it's cracked:

Now I'm exploring options for repair. I'm thinking of getting some plastic cement like Revell Contacta and applying a tiny amount to the crack to reseal the plastic.