With Angle Auto Pilot it will attempt to hold the angle you set but if set too steep for a plane (such as 2K/min+ in most bombers) it will nose down due to loss of speed. Auto-Climb will nose down when enabled to gain speed (this mode has a pre-set/manual speed it will climb at) and will do what it takes to reach that speed.
Just try a less steep angle before setting the Angle Climb. If it continues to nose down when you have plenty of speed maintaining the climb (150Mph or so or more) it's possible there is a bug or calibration issue.