Webflow automatically adds the following script to the <head>
<script>
! function(o, c) {
var n = c.documentElement,
t = " w-mod-";
n.className += t + "js", ("ontouchstart" in o || o.DocumentTouch && c instanceof DocumentTouch) && (n.className += t + "touch")
}(window, document);
</script>
Changing the above CSS to background-attachment: fixed; makes the desired effect work again.
You might be able to override the behaviour by adding the specific CSS. I’m not totally sure about the effect. There’s probably a good reason for Webflow adding this.
Have a look at the following three threads below.
You might have to do a bit of restructuring in order to make this work without adding custom code. The comment from @cyberdave might be a good starting point going forward.