I do have a slow travel speeds to fill gaps (not a real good fitter).
The above might explain why. I'm guessing you are trying to do all of this in 1 pass.
I do this too (but it's wrong). 2 passes is the better way. What is happening is that as
you join the 2 pieces together, you are thinking to yourself that this bead is too small for
a finished bead, so you stay there and add more filler wire. The bead instantly cools
because there is now more metal there, but your arc has not changed. So you give it more
pedal to keep the molten puddle going. But now there is too much heat, and the puddle
sinks into the joint. And you might even add more filler rod to correct for that. So what
you end up doing is trying to run 1 giant bead in 1 pass. Again, I do this too (but it's wrong).
Run a small bead to just join your pipes, then do a 2nd, or even 3rd pass.