I have 4 robots finished. 2 go reasonably straight, 2 do not. One motor glitches in each, lots of glitches in the 2 that curve around. It seems soldering with low temp solder at 200C makes a good motor. Motors soldered at 350C can't be fixed, only replaced. Doesn't take too long but is tedious.

My explanation that could be totally wrong. Flux removes oxidation from metal (the barrier to metal in solder). Flux binds with oxygen on metal and carries it away. But higher temps increases the oxidation in metal. Flux also coats the site, preventing atmospheric oxygen from binding.

Its actually crazy complex. Flux is 4 categories of chemicals, activator, vehicle, solvent and additives. Rosin itself, has organic acids for activators and vehicles that carry the bound oxygen away.

I don't know the metal of the magnet wire, or the metal of the pin of the motor. Different percentages of metals, and their porosity come into play pretty drastically, plus impurities present from where they are mined. With low temp solder, I introduced silver.

Crazy complicated, with metallurgy and chemistry. It can be understood, but is a deep subject.