Thanks for this info which I found while searching for the cause of this same problem.
I had made test samples using Aqua Magic on top of Liquitex Basics acrylic paint applied directly to extruded foam sheet and obtained the same short straight rumpling of the surface. It was worse in the samples where some Wickes emulsion paint had been mixed with the Liquitex.
if Sculptamold or Hypocal Light was spread on the foam, then the Aqua Magic on Liquitex paint was perfectly ok, so I am guessing that the foam reacted with the Liquitex in some way, causing the problems with the Aqua Magic.