The SERVO motors may not be selected to be ACTIVE, but the MSFS AP PID logic may still be running when the AP is turned on, and can still accumulate errors Then, without any resultant correction and reduction of those errors, the the PID integral term can still INTEGRATE up to a maximum limit, resulting in a large error, that later will be applied to drive the servo motors, when they are eventually turned on.
What is real life, is NOT necessarily modeled and simulated correctly in the sim - but it is getting there, as these issues are discovered, and corrected within the MSFS simulation.