It happens when you heat it too fast. Plexiglas is Polymethyl methacrylate or short, Acrylic as you first posted. Lexan is the brand name for Polycarbonate and is very hard to thermoform without bubbles, also the material that is known to yellow. American made Acrylic should not yellow or haze. It can oxidize but you can also use a deoxidizing solution to quickly clean it up. I think it should hold up and stay clear.