I had this happen, too - it's almost like bloom and glare has different size/intensity settings for a few frames, then it goes back to normal. I have a 300 frames animation and bloom&glare is unstable for 10-15 frames, then goes back to normal, then again becomes muted for a few frames, back to normal. It's almost as if some extreme value within the frame (maybe from an unresolved hot pixel, or maybe even a negative value) triggers it into a muted result... When inspecting the frames where this happens I don't see much difference but maybe you folks have some better way to analyze them. I can provide some frames for you (can't share the scene unfortunately).