a321 autopilot help

Ok here's my deal with whats going on with the a321. I set my autopilot stuff before take off autothrottle to 250 kias, and attitude to 25,000 ft at a rate of 1800. This is how I normally do it in the 737 and 747 is that when I take off and have a good rate of climb I press z to engage auto pilot so that the planes climb is smooth as possible. The problem I have when I do this with the a321 is that the plane doesn't seem to recognize the rate of climb. Instead it just goes to the max angle it can climb at. The only way I can quickly remedy this is to press the climb rate know to level off, at my current altitude then reset my desired altitude then the plane climbs at its correct rate of climb. Now I dont know if this is a bug, or if there is a reason for this kind of operation in the airbus. Can someone shed some light on this one for me?