I'm rendering a scene through backburner, and at some point in the render (random time), several trees are rendered as billboards, but not all. At first only occurs in one node, but soon start to "propagate" also to the rest. The billboards are retaining the material of the original object. All the trees are geometry (no proxy) and the renderer is mental ray (max2012). The same renders fine without backburner.
I'm placing the trees in custom edit mode.