erg mode inner workings

hey, anyone is aware how erg mode is controlled?

I use tacx vortex with stages PM. my vortex is highly inaccurate, measured ftp 301w when it was 260w (did not have stages then). the problem I’m facing now is that in erg mode wats are kept way lower then they should, in intervals when they should be 350w vortex hovers at around 310w. my suspicions are that zwift sends watts to vortex instead of instructions to lower or increase resistance which renders erg mode pretty much useless