I'm trying to render this lightning animation, and all the materials looks right (or at least how I want it to look) starting at frame 44. Every frame I render before 44, the materials render differently.
The lightning is made of 3 curves. The centermost curve is the most thin, with an emission surface material to brighten up the second curve, which is slightly thicker around the first one. This one has a blue emission volume material. The last curve is a very thick curve with a blue surface emission material, and is invisible to the camera. This curve is just to provide some glow on the ground plane, as the thickness of the other curves is too small to actually illuminate anything in the scene. The lightning is animated by keyframing the bevel factor of the spline.
So what's going on here? There are no keyframes in the materials to cause this change. It looks like two of the curves might be overlapping? But how could that be if their bevel depth isn't animated? I tried turning off the motion blur, changing the camera angle, changing the bevel depth to make sure they aren't overlapping. No success yet.