Highlighted part actually doesn't make vein grow vertically.

Why?

Because there will not be molten ROCK with tmp = 1.

Every 'tmp=1'ed particle is now molten GOLD, not molten ROCK, hence that part don't have any effect.

So This is why GOLD cluster is not trended vertically.

====edited : improvement?====

This is what happen in current snapshot

(Air simulation : no update, pressure : 256+, temp : 8000+, from single pixel of molten GOLD and molten ROCK)

(Used simple LUA mod to visualize : Blue pixel is molten GOLD, red pixel is molten GOLD with tmp = 1)

It is intended to have 'vertical trend', but it just looks circular.

I changed a code little bit:

and this is result:

It trends vertically (as intended, in my opinion)

====Edit====

This suggestion is now applied to main github repository. (Commit log)