You say they worked on the tail rotor only during the 25hr. I do not know the 412 at all, but on a EC product we have a colective/yaw coupling. The S76 has similiar couplings between her control channels affecting each other. If the 412 has a collective/yaw coupling I would check the tail rigging and/or assembly as it might be inhibiting the collective travel at higher rpm due to it's pitch setting adapts as the torque increases.
If no collective/yaw coupling-guess it can't be the tail then.