Depending on the card it could be due to a setting triggered in the BIOS to overclock the card a bit to drive the display. You should be able to overclock the card back to it's original performance.
Thanks for that, I decreased the core voltage a bit and its mining 30.4mhs again!
But a little increased wattage
I have now 2 cards running RX 580 nitro+ 8gb
Dual mining 60.7mh/s and 2040mh/s consuming 385W (whole setup off wall)
Is that normal?
Optimally, the GPU shouldn't pull more than 90W when mining, you can use software such as MSI Afterburner to view that. Keep in mind that it doesn't account for GDDR5s power consumption so the total power consumption of the graphics card is significantly higher than what software may display.