There are two UDNs related to this issue. It makes the Path Tracer unusable with MGPU, with atmosphere enabled.
I already have a one-line fix implemented and tested. The atmosphere scan line artifacts are caused by the atmosphere lookup texture not being generated on all GPUs, so it just needs an RDG_GPU_MASK_SCOPE where that happens.
Steps reported on UDN:
Observe scan line artifacts across the entire frame (helps to take a screenshot and zoom in).
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-176829 in the post.