On second look the threaded hole just runs WAY deeper than it needs to. You know your stuff.A square taper axle has a threaded hole. I vote for bad heat treatment or flaw. Watch. some Forged in Fire episodes. LOTS of ways to produce a s*it hardened steel part, destined for failure.
Golden motor lists their kit at under $100. I'd have little faith in the quality based on pricepoint.
DId you use a torque wrench to install the axle bolts?
Yeah I used a torque wrench and 2 step tightening. Tightened by hand, rode a few hundred feet then torqued on. Learned that after I ruined the first set of cranks. These things are a lot lower quality than regular bikes, that's for sure. I ended up with treated steel unicycle cranks in the end. So much better.