JavaScript to change image

I'm definitely with you on the benefits of slide masters, and I have a lot of success putting code on one master so as to invoke it on subordinate instances. I guess that's why I'm wondering if there's another factor I'm not considering.

To test, I imported a stock image and gave it a second (non-Normal) state that reversed the image.

Then on the start of a slide master, I added a JavaScript trigger that captures the image object:

const img = object(id);

And then if the mouse is on the on the left half of the slide, show the flipped state of the image. If the mouse is on the right half of the slide, show the normal state of the image:

if (pointerX() < slideWidth()/2) {
    img.state = 'Flipped';
} else {
    img.state = 'Normal';
}

This kind of code works identically on slide masters as well as lower-level instances, which is why I suspected maybe there's some other aspect of changing an image on a slide you're encountering.