bDynamicBannerTransparency=false was responsible for it. Now it's fixed.
But regarding the question. There was 1 instance where it turned black - just sky and siege flag. There were multiple instances where it goes disco for prolonged amount of time, i did screens but it's easier to said that movable props make it kick in quite often (gate wheel, siege tower)(occluded by other models or not).
New client, same issue. This time around, if i stand in one place its stable dark or light. dynamic banner transparency don't solve it this time around.