LTSpice - envelope detector LPF behaving like buffer
Here's an envelope detector:-



What you have is a diode on the input of an op amp which has bias current of 10fA (0.00000000000001A). At this current the diode is practically a short circuit.

To work properly the envelope detector must have a load. The capacitor needs to be much larger than the diode's parasitic capacitance. The resistor needs to draw much more current than the diode's leakage current. It also has to be low enough to discharge the capacitor during the troughs of the modulation, but high enough to follow the peaks of the carrier. These values are a compromise between carrier ripple and modulation distortion.