Weird. You could try soldering in a piece of copper rod/ thick wire/ soldering braid to reinforce it.
If it were a much thinner solder trace then I's say it was over-current ( is a built-in feature on some lipo packs, designed to act as a fuse of sorts.. ), but a trace that fat... weird....
That fat trace is to lower the resistance a bit between the two connection point. I went through the same issue a few times and fixed it by adding braid wires.