Autopilot

I followed a lot of tutorials on YouTube and they all do the same thing and I do that and everything is correct because it’s basically the same as starting up the a321lr and I can do that but with the A320neo the flight plan and everything is set and when I takeoff I put it into autopilot and I turn off my controller because I don’t need it anymore until landing and then around 10mins in the autopilot disengages and so does the flight director and when I turn it on again it just points up to around 40000ft and then dives down and does its own thing. I do the exact same thing with the a321lr and use simbrief for both and it works perfectly fine with the a321lr. Any idea of what I should do?

Edited December 26, 2025Dec 26 by Msfslover