When you make the Go Around Check Thrust Flap 6 call you are also pitching for the go around attitude. The config change from flap 30 to flap 6 only causes a sink if you don't change the attitude/AOA at the same time. Checking that the thrust is set makes sure you have enough energy going into the system such that the pitch change means you don't lose speed. Once the aircraft is established in the go around attitude you don't need quite as much lift anyway which is part of the reason why the small reduction of lift from retracting the flap from 30 to 6 shouldn't cause a significant sink. That would only happen if you don't change the attitude..
well thats what I think.. could be wrong.. who knows, its all witchcraft