Yes, attitude mode is the only manual vertical autopilot mode in the aircraft.
Very strange. There is nothing that should interfere with the operation of normal trim, autopilot or not.
Unfortunately, all I can say right now is to let me know if it happens to you again, and we can try to troubleshoot together. Since autopilot functionality is often tied to the custom autopilots in the GPS addons, pay attention to which GPS unit you are using if it happens again, or see if it happens with all GPS units.