Is the servo moving backwards?
Bingo! Check the servo direction. Engage alt hold on the ground and lift the tail of the aircraft up. The elvevator should move up. I suspect you'll find the servo moves backwards. Check your installation manual for the pitch reversal instructions, it's in the wiring diagram. Thanks!
Snipping the yellow wire loop fixed the problem. The ALT Hold is rock solid now.
Still have some configs to adjust, such as magnetic calibration, bank angle, etc.