swiper slider off-screen images not "preloading"

[nosilver4u]

Describe your problem/question
We've run across a strange issue with a horizontal slider/swiper where the images don't get lazyloaded until after they come into the actual viewport. So far as I know, anything within the "expand" threshold should be loaded right away, but this is very obvious that the images never load until they comes onto the screen (excepting the handful that are on-screen from the start).

Make it reproduce-able
This is on a client site that we didn't build, we're just providing the lazy load and CDN stuff, but I've slimmed down the page so the swiper is right at the top here: https://codesandbox.io/s/mystifying-butterfly-xmihcc?file=/index.html

Add tags/keywords
expand, viewport threshold

Is this perhaps some quirk with the way the swiper is using the transform/translate3d effect to move the images? And is there any way you can see to improve it?