hullaballoo,
"The engineers may be on top of this, but gear boxes shouldn't crack"
You are spot on. An MRGB housing that carries rotor loads is a crit 1 piece of structure. As such, the casting must undergo class 1, grade A or B, 100% radiographic inspection at the foundry, with very tightly controlled acceptance criteria. And the part is also analyzed with a very conservative FoS to boot. Under these tightly controlled QA and analysis procedures, a casting structural failure would be highly unlikely.
If the failure occurred at/near a structural attachment point, I would suspect that the root cause is more likely due to an improperly installed fastener that came loose and created excessive loads on the remaining attach points.
Of course, this is all pure speculation on my part. So take it for what it's worth.
Regards,
riff_raff