hey ! never heard this, can you tell more details ?
we practise single engine airwork as well single engine landing on the cheyenne with pt6a without any issues. of course always first bring throttle back to idle since when you feather under load the big resistance from the gearbox may truly screw up something.
when you pull the props forward again ( eg. at touchdown with one prop feathered) they will kick in quickly , but never observed an overspeed.
my only quess would be that he had not the power lever at idle when he unfeathered the prop again and the power output of the turbine created such a massive spool up of the prop/free turbine stages that the governor was not able to stabilize and a destructive overspeed happened.
can you find out what exactly failed on the engine ? the gearbox? the power section?
cheers !