Okay I have found a way. Because we can't undervolt via Claymore, I change voltage value in bios, then flash, in order to bypassing Claymore undervolting ^^
Did it work on you? What miner version are you using and what AMD driver?
It works. I have set voltage and frequency with PolariosBiosEditor, flash it with atiflash. Then, I use AfterBurner to preset core/memory frequency (boot with windows), and HWmonitor or any monitor read my settings : 1266MHz/900mV and 2250MHz/900mv
On the wall I have around 720W with my 4 RX580, like before blockchain driver, and everything running well.
But I still waiting claymore update to try lower voltage without flash bios at each change ^^'