Mottling in areas of rendered video

After rendering a couple of different projects on DR, I've encountered an issue in my most recent project where one particular set of scenes shows noticable mottling in darker areas of the image. It isn't there in the original AVCHD and H.264 source footage from the 2 cameras. I'm rendering with an H.264 codec and tried both MP4 and MOV containers with no difference between them. Tried bitrates from 8k to 12k. An image of the effect from one corner of a shot is attached. I've tried a number of different render options but none of them seem to be affecting it. Wondering if anyone has encountered this before and can point me to a likely culprit. Any suggestions welcomed!

Thanks,
Jeff H.