@volkaertskristof I tested it with my button, and the behaviour is not the same as you described.
When i hover over the button (which should get —> “red” on hovering), instead nothing happens and the color of the predefined “wix-UI” is shown.
I can even hold on it 20min. and the color do not change, like it should do on → onMouseIn().
Here the example… https://www.media-junkie.com/blank-2
$w.onReady(function () { });
export function myButton_mouseIn(event) {$w('#myButton').style.backgroundColor="red"}