Very hard to tell from just one picture but looks maybe like the two lines are just sticking, not welding.
Are you using shrink tubing? Dumb question, I know. I only ask because I can’t really see some of the tell-tale signs on the pic, but it is super grainy on my screen.
Cleaning the line surfaces well beforehand?
Also, just to be sure %100 sure - the stuff you are using for the loop? Is it the same stuff - chemically I mean? Different materials melt at different temperatures, and some stuff can’t be welded at all. I’ve done this particular type of weld using an extra piece Rio
ridge shooting line. Can you make a consistent good welded loop with just the random fly line to itself - when finished there should be no visible “seam”.
What do you have the temperature set to for your hair straightener?
Anyway, a first guess would be not enough heat applied for consistent full liquification of one or both lines, and/or enough time where both lines were in a melted state. The related culprit might be different and incompatible materials. Well welded lines should not come apart like this, along a well-defined boundary between the two pieces, and OVER heated stuff (harder to do with the iron) looks different usually but having the original line rip apart, that is usually when things got too hot. Was it that way right out of the shrink tubing?
FWIW I’d use a bit more material for the loop and weld PAST the line a bit, so the extra line is welded to itself for a short section.