Looks like too much heat. Start the arc and let the puddle grow slowly. Be patient and don't rush it. That thin of a wall tubing being welded together via butt splice is something you wouldn't really ever do outside of just practicing, as you'd need a slug to maintain any really sort of structural usefulness, so I wouldn't stress over it too much. More of an learning exercise than building a real-life skill. I don't mean that to sound rude. It's good to practice on difficult joints to get your confidence up. Heat it up slow and get some filler in there pretty soon to bridge the gap that will open up as soon as the edges start to puddle. As flat as your other practice bead is, I'd say there was a lot of heat involved in that weld. Very nice looking bead! But it does look a little overly hot. With that in mind, it's possible you're coming in too hot on that tube joint, which is why I mentioned it. Good luck!