I stuggled for a long time welding vertically.
I started off going down hill not very successfully, little penetration with the weld pool running all over the shop. Then tried upwards and found it much better going uphill. I can do either now but prefer uphill
It was a bit of a surprise struggling with vertical. On thin gauge stuff I'd never noticed a problem. Vertical, upside down, whatever. Didn't make much difference. Probably because your going quick and trying not to add too much wire/heat.
But trying to pile weld into a vertical weld on 5mm angle, different ball game.
I'm looking forward to trying some more so I can get it sussed.