VP
There's no myth that you can't take a FADEC engine to idle when in flight. The problem lies with the bits in between idle and flight rpm when the pilot wishes to reduce the T/R RPM partially to reduce its thrust.
TC
Another helicopter that drives the T/R from the engine is, wait for it.......... pehaps the most prolific light turbine engined helicoper ever produced, the Bell 206 Jet Ranger.
So it's not peculiar to the AS350, far from it.
JJ