In FLCH, the autopilot (or FD, if AP not in) will try and keep the commanded speed by adjusting the pitch/pitch demands. If you don't follow the FD in manual flight, you won't get speed protection as there is no 'wakeup' in FLCH.
Summary: If you disengage the AT then pull the nose of the aircraft up in the air and leave it there, the aeroplane will slow down towards the stall. This is normal behaviour!