The culprit is MM enamel. They have tinkered with the paint formula for the worse. I had the same problem with MM flat black. It was like the paint no longer thinned properly with lacquer thinner. It might also be the lacquer thinner formula changing to a more California-friendly mix, as my old can didn't have any problems but the fresh new one did.
Okay, so what works? Actually, the MM Airbrush Thinner seems to still work. I also tried using Mr. Color Leveling Thinner and it also worked. HTH.