How altitude changes with changed QNH? How the AGL altitude changes with changed QNH? I'd like only to confirm or correct my intuition..

Airplane is maintaining certain FL with some QNH, its instruments are showing some AMSL. Now the airplane enters area with lower air pressure, but keeping the same set QNH. How the real AGL altitude changes?

My intuition is that altimeter is effectively showing certain pressure. If the atmospheric pressure drops (e.g. from 1013 to 990), to show the same measured pressure (altitude) the airplane has to descend.  Then the airplane is having lower real AMSL than displayed on instruments.

Is my intuition correct?
Yes. There's an old saying: when flying from high to low, look out below! https://sabushpilot.com/high-to-low-look-out-below/ has nice properly descriptive image.


  Airplane is maintaining certain FL with some QNH


Note that if you were at a FL (flight level), you wouldn't be using QNH, you'd be using QNE. You use QNH up to the transition altitude and clearances will be given to maintain a certain number of feet (or meters in some countries). Above the transition altitude, your clearance would be to maintain a certain FL and you would be using QNE, 1013.2 hPa.