It's normal for the engine to be able to make power above the MAP limit on the placard without drooping rpm.
The torque produced from a "standard" engine varies with density altitude (pressure and temperature).
The MAP gauge/OAT/placard combination is a poor mans torque gauge. Limiting the torque ensures the components (engine / gearboxes) go the full 2200 hours.
Pulling past the limits on the placard is applying more torque to the rotor drive system than what is approved... its bad airmanship and not necessary with good planning. The engine and gearboxes will cope with the increased torque but may not last the full 2200 hours.
Not all engines are created equal however... they do not make the same power (torque) at the same indicated MAP. An indication of the health of the engine is the MAP when you are on the ground at 102% with the pedals neutral. Where I am (usually ISA +25) a good engine will indicate 12-13, a bad 15+. It's a measure of much power its taking to overcome the drag and friction of the rotor system which is fairly constant between machines. Learn what a healthy engine produces for your conditions and check your MAP every time you wind up if you want to avoid surprises when taking off.
There are so many potential issues that make an engine a "Bad engine" e.g. low compression, loose plugs, bad leads, bad fuel, magneto, blocked air filter mixture... if its drooping at less than the placarded limit (with the throttle fully open) I would suspect you have one of those.
If its drooping and the throttle isn't fully open then it might be the governor actuator is a bit tired.