I was not aware of the AB139 connection, but as a newer programme its track record would be less of an indicator than the old BK117.
So there must be another cause. I am no expert on gearboxes let alone the differences [if any] between the BK and the MD MGB but that leads us towards asking questions about design differences relative to the interior of the 900 MGB. Are cogs just cogs or have they changed materials?
Another train of thought might explore the differences in the dynamic forces affecting the power train of the 117 and the 900.
You do not see 900s '...do a lot of heavy lifting work...' and yet the 117 is said to be pretty resiliant against such abuse.
Does anyone have any further ideas?