I had the same issue with wattman while trying to undervolt my rx 480. Solved it by changing the clock speed. I had 1310mhz stock and wattman kept putting voltage back to 1150mv on restarting.
It only did this with power state 7 though. ?
Anyhow, I dropped the core to 1300 and wattman kept the voltage at 1040mv. overclocking or underclocking, even by 1mhz resulted in wattman keeping the voltage I set.