When a oil tube joint is found loose, it would be natural to assume it wasn't tightened properly during original assembly by the assembly mechanic. That is the obvious and it could be true. It may not have been that it was the assembly mechanic, not tightening it to specification, perhaps his torque wrench was out of order, so the wrench was the culprit. But the history of this particular engine is not be so clear to us. The engine was originally assembled sometime in 2008, so it probably was a Mod A. So during its life, so far, it was probably disassembled and reassembled at least once and maybe twice in going from Mod A to Mod C. But then again, maybe this particular joint wasn't involved during these changes. But then again, there is the not so obvious.
Could it be possible that as a result of making the changes from A to C, a vibration problem (noted in QF32's stub pipe failure, at least partially due to fatigue instigated by vibration) has moved up stream, so to speak? In simple terms, is vibration still affecting an oil supply line to bearings at this engine location, only now on the outside of the engine? Did a properly torqued oil line connection vibrate loose over a period of time? A real possiblility, we will have to wait to see.