I beleive this to be a classic case of poor mechanical installation and over reliance on electronic end point adjustment. There is nothing wrong with your radio. This does not allow the servo far enough down in the range where the the throttle trim will function.

The mechanical advantage of the pushrod geometry is way to much, The pushrod is connected to the servo output arm to far out and possibly the pushrod is not in the furthest hole out in the throttle arm. This forces the low end point to be so low that the trim is effectively bypassed.

Try first moving the pushrod to a hole closer to the servo and make sure it is in the farthest hole in the throttle arm then open your high and low throttle endpoints up to 100%. and start over.

John