Surface Tension aside, I believe that common sense should prevail and one should be able to understand when you are alloying material the slag, gangue or flux would be forced (pushed) aside and that’s not even considering the state of the flux being any where from a plasma shield to a gas.
But, in all seriousness should the flux (shielding) attempt to rise (as we know it would), compared to the heaver metal where would it go?
Now if someone ask why sometimes the flux stays and the puddle falls down, I can think of lots of illogical explanations but none of the ever work.
Why can't you push a rope??? Seeing as how its so much lighter it ougth to be easier than pushing a chain.