It looks like a rendering bug in chrome and I'm not sure there is a real fix for it.
I can tell you the cause and you may be able to work around it. The trigger is the position:relative in .story and you can test this by adding position:static here:
<section class="story" style="position:static" id="story-lowerys" data-controller="Lowerys" data-scrolloffset="100">
The nested element .bg is placed absolutely but you may be able to getaway without a stacking context if you avoid using co-ordinates and instead use margins to move it around (I tested locally and the left:100px didn;t actually seem to break it but I wasn't working with the full site).
Hopefully that will give you something to try anyway.