Today, I bought 2 more Sapphire RX580 with Hynix memory, they are both run about 22mh/s, and I tune/mod bios,they are still 22mh/s.
try a different motherboard, I have an older motherboard that does the same thing on my rx400 series while flashing/overclocking them (22MH/s), but when moved to my mining motherboards they run fine