motion script: ease function doesn't work as intended

I wanted to link an animated property to a marker.

This is the property keyframes that I wanted to map according to the marker.

I used this expression and I was expecting it to work properly.

var markerIn = thisComp.layer("Animation Markers").marker.key(1).time;
var startTime = markerIn-0.25 ;
var endTime = markerIn+0.25;

var valueMin =transform.opacity.key(1).value;
var valueMax =transform.opacity.key(2).value;
ease(time, startTime , endTime, valueMin, valueMax);

However, there is weird issue when it comes the final output of the animation.

The interpolation of the animation returned by the ease function, starts the animation way too early as you can and not from the startTime value, as you can see it starts after a few frames and I can't find any correction for this issue.
I appreciate your help if you can help me fix this issue, and thanks for passing by and reading this thread.