Took me awhile to reason out the support tube bowing.....
To bow that way, the top of the support tube has to shrink more than the bottom.
Looks like you welded from one side to the other. While welding the top expanded more than the support tube. So it grew a little bit in comparison to the support tube as you laid the bead. Once cool, the top shrinks, bowing the support tube.
I think you can avoid bowing if you tack one side, then the other let cool. then do the bead.
