Moving Position on load? (background-attachment: fixed)

Hello,
When adding background-attachment: fixed to my hero image, the starting position moves when the page is loaded (see GIF below)
https://gyazo.com/b957d488dfdb4ff09938272699915a65

How do I prevent this from happening? Do I need to offset my background image? Here's the rest of the CSS applied to the img (div)

.hero__img {
    position: absolute;
    background: url(img/char.jpg) no-repeat;
    background-size: cover;
    background-position: right 35% bottom 24%;
    background-attachment: fixed;
    width: 100%;
    height: 100%;
}


This issue goes away once background-attachment: fixed; is removed

Thank you
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