It looks like Sim update 13 has added a bug in which the hydraulic pressure drops at ground idle with the throttle. Probably due to the new propellor code. Unfortunately, nothing we can do but wait for an update to the aircraft. For now, I have the throttles a little higher to make sure I have hydraulic pressure.
Some more chat here… ATR 42 / 72 hydraulic press. and some electricals go out after landing when engine IDLE - Bug Reports / Aircraft & Systems - Microsoft Flight Simulator Forums