Have you checked the quality/performance of your RX 570 with any stress test software (like FurMark) yet? If no, I'd suggest you to do that. If your GPU passes the test, it'll confirm that the problem is with software (like Windows/drivers), not your hardware (GPU).
I guess I will try that. Restarting computer worked with that blockchain driver I was suggested to try but no difference in mh/s at all.