Do you have AI piloting turned on?
The work around I found was too bind the autopilot disconnect to a key/button and then use this instead of just deselecting the autopilot.
I believe when there is a missed approach programmed into the flightplan and you just deselect the autopilot it tries to do a go around.
Alternatively make sure there is no go around programmed and it may work with just deselecting the autopilot.