Are you planning on using MR for your final render? If so, then why wouldn't you use it to conduct the test renders? Iits not like MR shaders, FG and caustics are mandatory when using it.

It seems to be a problem with the file format, don't know whats up with png I rarely use it so I'm stumped. I tried .jpeg and I still got artifacts even when it was saved at max quality. Saving it as a .bmp solved the issue.