Spalling
Agree that the Bell pinion photo is relevant, but it may have helped initially if it explained that it was a different product.
Spalling is a quite normal "wearout" mode in gear or bearing designs. It is actually a form of metal fatigue itself at just below the surface of contact due to contact stress. It can initiate at asperities, or even surface damage from dents, handling, or debris going through the system. It progresses from smaller pits into larger flakes. Often a small pit, or beginning spall, will not produce a significant chip enough to be indicated, although they will usually stick to the magnetic detector commonly found on most aerospace transmissions. The progressive nature of the spall produces more chips, which will accumulate, and bridge the gap on the detector. My 50 cents guess was that the one in the photo was detected in such manner.
Spalls will progressively worsen in the direction of the contact, which also means that examination can sometimes pinpoint the initiating location, such as a dent.
Issue with the subject mishap pinion is that the raceway spall produced high stress concentration enough to produce a secondary fatigue made, which became catastrophic. Probably was not ever predicted.
If someone were so inclined, they could compare the rim thickness of the two examples, as a ratio of tooth size. Suspect the Bell version is inherently thicker, thus more stiff, thus less relative bending stress. Thus, less likely to experience a secondary rim fracture fatigue. Now of course that is 20,000 foot view, without any real calculation of stress due to load. There should be a stress level that the 225 pinion should be able to tolerate, even with surface spalls present. What makes this questionable now is that the local stress is probably higher than predicted due to unknown reasons, and it may be that by the time the questions are answered, it may point out that the gearbox design is over-rated for power capability.