I have the same issue with 8 RX 570 cards.
I used to mine ETH in December with about 26 Mh/s per card. I switched the rig to mine Cryptonight until few days ago and when I tried switching back to ETH, surprise - maximum 17.5 Mh/s per card!
Anybody has any idea what changed? My cards are having BIOS modded... everything worked ok until now. model is RX 570 ASUS Strix 4GB (ROG-STRIX-RX570-4G)

Thanks a lot!