a plane mesh in plasma acts a bit weird from time to time, depending on what kind of component you have on it, for instance a plane mesh depending on the terrain setting it has may extend beyond the mesh itself producing a unintentional proxy terrain.
or using them at an angle like for a stair case or ramp it has to be set to exact which uses up more resources. For stairs or a ramp if I need a proxy terrain I usually use a box set to an angle and have the terrain component set to hull. A plane with one face set to exact or a box with six faces set to hull six of one and a half a dozen of the other