I too haven't really seen acrylic behave this way, only when it's been sitting for a long time and has separated a bit but then still only needs a good stirring to bring it back. I even popped the tops off the bottles and used a stirrer to make sure there wasn't a clump of paint down in the bottom of the bottles. I made a little comparison pic to show what I'm talking about as it's kinda hard to describe on a piece of scrap plastic.