Animated background gets lost if article has vertical class

[pwa-tapptic]

Hello guys! I encountered strange issue with animated background. Here are my slides:

<section>
  <div class="wrap">
    <h1>Simple stuff</h1>
  </div>
</section>
<section class="bg-blue">
  <span class="background anim dark" style="background-image:url('https://source.unsplash.com/DIp71kvwXhU/')"></span>
  <div class="wrap">
    <h1>Glitches...</h1>
  </div>
</section>
<section>
  <div class="wrap">
    <h1>Simple stuff</h1>
  </div>
</section>

There are strange glitches on my animated slide when I scroll vertically down (so if my article looks like <article id="webslides" class="vertical"> and I move from top to bottom).
Everything works fine when I scroll up and my animated slide is a previous one. It also works fine if my article does not contain vertical class.
I checked on different browsers (Safari, Brave, Chrome), but the glitches are the same.

BTW Thanks for this awesome tool, I hope it will be still maintained and developed!