I assume you’re new to IF and the forum, so welcome.
The nose dive happens because when you activate the A/P, your elevators assume a different position rather than the neutral when before activating it, so they’ll rapidly go back to that position when turning off the A/P.
To counteract this, use the TRIM. IRL they ease the pressure the pilot needs to make on the yoke to keep the nose of the plane up.
The benefits of this are that you won’t need to recalibrate if you remember more or less your original device possition when you began playing.
For example, on the Boeing 737-700/800, I use 28% of trim and push my phone down to avoid having my plane go up (I’ll have to revise that number, try 27%), the 747-400 needs 17% and the 717 uses 20%. The A318, although it doesn’t have the nose dive, uses a 5% trim to have a nice transition.
All of these planes use Normal weight, and IRL they don’t nose dive.
Hope this helps.