Pitch Response...
Actually, the FCTM is quite specific and brings attention to flight crew in regards to use of the speedbrakes while the autopilot is engaged. If the autopilot has captured the selected MCP altitude (which can be 2000 feet above depending on descent rate), the autopilot calculates a fixed descent profile to intercept the altitude. If the speedbrakes are are 'stowed' during the maneuver, the autopilot will pitch down to compensate for the drag loss, resulting in an overspeed. With the resulting overspeed the autopilot reacts to envelope protection and will level off.
The proper method of retracting speedbrakes during this process is to intervene with an MCP speed of about 20 knots less than what was being indicated, prior to speedbrakes stowage. The aircraft will begin to slow down and when the speedbrakes are stowed, the autopilot will command a smooth transition to adjust for the speed/pitch. Works like a charm. It is not a design fault but a characteristic of the autopilot system and the low drag profile of the airplane. If being held 'high and hot' is a problem, inserting a speed/altitude on the legs page, down track will provide a good 'how-goes-it' with regards to a vertical profile during the descent.