definitely design issue. The DR being such a detuned overbuilt engine should withstand any abuse at the gearbox.
The regular occurrence of the similar faults is evident here..
You dont usually see pistons, rings, cylinders or cranks failing on DRs, they all seem to be 2nd or 3rd gear...
"And some guy said, let there be filtering, and we filtered and we saw that it was good, and hours became minutes..."