A lot of highly stressed shafts etc (like in a transmission) are made
of AISI 6150 steel which is probably similar to what Lycoming
specified. I understood that Lycoming also nitrided their finished
shafts, which should further increase the fatigue strength as well as
provide an excellent wear surface on the journals. (The hinges of hell
are nitrided.)

As a Mech Engr I thought adding Vanadium did only good things to steel.
I don't see how they could save any machining costs by adding
vanadium. Surely they didn't add vanadium just so they could skip
nitriding?

It is interesting that it appears to be only the turbo models that are
involved. I wonder what the condition of the harmonic damper
assemblies and their associated pins and bushings etc was for each
failure.

I'll bet it is appealed..... I sure hope so at least.