Looks like it could be just simple bad design and overheating of the trace. It occurred just where the trace necks down, where the big copper plane can’t pull away heat anymore and before the heat sinking effect of the diode’s beefy leg.
If replacing the diode fixes the problem you can use a beefier wire than what that trace provided so there isn’t a heating choke point at high current levels anymore.
Check around for other signs of heating and beef up whatever you can there too. More solder can help a little but it’s 10x the resistance of copper so a lot of solder only helps a bit.