turn setting down a touch or.move faster. Undercut is due to too much of the parent metal.melting and being drawn into the puddle. All about heat control, keeping the puddle hot enough for penetration but not too hot that the parent metal.suffers.
Can you mark where its undercut as i can't see it, looks to me as if the bead is lying on the metal and not cutting in to the sides at all or am i misunderstanding undercut, i'm new to welding and ask so i can watch for it if it happens to me.
thanks
Personally, I wouldn't call the circled areas above, undercut.
Saying that, I have had no formal welding training, nor am I an inspector, nor can I perform a V-Up with mig very well. Just looks like you've moved a little fast.
looks cold to me as there are absolutely no heat marks on the weld or surrounding plate but as i said i'm just learning. A cross section would have been nice.
You prevent undercut by pausing at each side of the weave. Count, one,two across, one,two across or one,two into the corner and out, one,two across, one,two into the corner and out. Keep practicing and you will develop a smooth consistent rhythm (without counting) and no undercut. Poor prep can also cause undercut, so keep your plate nice and clean.
And yes that is undercut. When I was training 10% of plate thickness or 1mm max over some of the weld length (cant remember how much) was allowed. I don't think many places in industry would allow 1mm deep, of undercut to pass though.