All I fly is the X-Plane 11 default Beech Baron and you are describing what I am encountering since the X-Plane 11 update of the past month or so. Before the update, I could use the autopilot up/down buttons (which would disengage the "alt" hold light) and trim to the climb/descend rate. Then, once I achieved the new altitude I could push the "alt" hold button, the "alt" hold light would come on and the Baron would level out and hold the new altitude. But since the update, what happens is the Baron autopilot refuses to let go of the altitude being held. I repeatedly push the "alt" hold button, the "alt" hold light is off momentarily and then the light comes back on. I try pushing the "down" trim or the "up" trim on the autopilot and it mostly refuses to do anything. Once in a while, if I keep pushing the trim buttons on the autopilot, the "alt" hold lets go. This new configuration is very annoying.