Background Transitions "out" not working on iframe

[demlak]

for example:

<section data-background-transition="convex" data-background-iframe="https://cataas.com/cat">
  <h2>test1</h2>
</section>

<section data-background-transition="convex" data-background-iframe="https://cataas.com/cat">
  <h2>test2</h2>
</section>

results in a slide-in of the iframe as a background.. but sliding to the next section is just a disappear of the first section and slide-in to second section..
but with convex we want to have it "carousel-like"..

tests show, that using attribute data-preload "heals" the not-sliding.. but we need to have uncached data.. so we can't use data-preload

Bug? wrong config on my site? wanted behaviour?

how to fix?