I was practicing my vertical up with mig a couple weeks ago. I was getting some undercut along my weld. It was small, but still something to fix.

My approximate settings were:
fillet weld vertical up
3/16" mild steel
.035 er70s6
c25 gas @ 25cfh

This was a couple weeks ago so I'm pretty sure those settings are correct. I was doing a inverted-v movement pausing for about a second on each side of the weld and making sure that the arc pushed as deep into the corner as possible. This was also done with a millermatic 251. I don't know how the softer arc characteristics affect a weld like this.

Based on what I saw I think the fix is a little higher wire speed and a little more pause on the outsides of the weld.