Problem with constraints in waypoints (A320 in roller coaster mode!)

I have a simple question/problem when following a flight plan with the A320.

When a waypoint has a height constraing like for example 9000-12000 the aircraft always selects the top value: 12000 even if the flight plan has that waypoint at 9000. Once the waypoint is reached then the autopilot dives violently to reach the lower limit, in my example 9000. And if the next waypoint is 5000-9000 it will stay at 9000 and after reaching the next waypoint dive to 5000 etc.

Is there something wrong I might be doing or a way to fix it?

Thanks!

Luis