Did it have a similar distance from motor mount to pinion as the MK4?
I’m wondering if the driver here might be that distance… If you think about pushing the shaft to one side (as the gear does as the shaft spins, it puts stress on the bolts on one side, while removing it from the other. If we assume a perfectly rigid shaft, then the amount of stress on the bolts increases the further away you put the force (t=f*d). And as the shaft spins, this stress oscillates - you go from full stress to no stress (aside from the normal stress from tightening the bolt). Oscillating movement/stress can cause material fatigue leading to failures.
And, to be perfectly honest, I’m hoping that it’s that distance that’s the driver, because it means other gearboxes, where the pinion is close to the motor mount, are going to hold up better and my robot’s going to be fine