I agree with JTO...The shaft was able to rotate with the rotor head enabling the nut to be compromised and shear the spilt pin to twist off. The issue with the bearing appears to be a lack of grease which was either burnt off or just not there when installed - that is not clear in the report.
In my experience similar TR pitch shafts have been mounted in splined guides to stop rotation.
I understand why the Actuator went to full travel and I'm unsure if a neutral default/detent setting is possible for an input failure mode.