Hi, Resolve is optimised for 16bit processing, and therefore all color grading should be done on source .cri for optimum performance, the sub sampling seen on a post processed 8 bit image is expected behaviour.
I chatted to the Resolve team about this, their recommendation is 10 bit or higher. Can you repeat your process on a 10bit render and confirm it looks ok, at which point that's as far as our help for you can go.
Let us know if 10 bits still has issues and I'll chat to them about it.