Same issue here.
Installed two identical GPU (Rog/Strix RX-480/8Gb) that seem to function Ok. With Crossfire turned off & GPU Core Voltage reduced.
Added the same Under-Volting to both video cards via Wattman & this is accepted and works for a while. But every now and then, about once per day, it drops out and back to the normalised, unchanged values. (With all undervaluing values reverted).
BUT.. this Only occurs on ONE of the Cards! However, after the reset, it contuses to work (but with much greater Wattage in this case).
I've observed no trigger, cause or concurrent-action to explain it. The other GPU retains it's values and is apparently unaffected.
SO, why is this occurring & how can it be remedied?