Why do hardware options appear to have little to no effect using PhoenixMiner (was also the case with Claymore) on Ubuntu?

They work fine on Windows 10 where I can get my Vega 56 cards running around 120 watts with a decent hash rate, but I can't bring down the power at all when running under Ubuntu? For example, the desktop I'm writing this on has a Vega 64 LE that I know is good for high 40s hash rates with ETH, ETC etc. But under Ubuntu I can't budge it from 36 MH/s and 260 W.

Currently running Ubuntu 20.04.1 with open AMD 20.30 driver and PhoenixMiner 5.3b.

Any ideas?