I think there may be general confusion in the MSFS community about the Autothrottle and autopilot. I’m not as familiar with the Airbus system, but with Boeing they are separate systems that work together. I should be able to engage and use the autothrottle system without ever turning on the autopilot. If I turn off the autopilot with autothrottle engaged, autothrottle should remain on.
The Boeing autothrottle can be set to maintain airspeed or engine setting.
I’m still trying to figure out how it’s working in MSFS. I can’t tell if it’s designed differently or malfunctioning.