Page where it works: https://jojowiki.com/index.php?title=Minor_Characters&mobileaction=toggle_view_mobile
Page where it doesn't: https://jojowiki.com/index.php?title=Unnamed_Characters&mobileaction=toggle_view_mobile
It also doesn't work on the home page https://jojowiki.com/index.php?title=JoJo_Wiki&mobileaction=toggle_view_mobile

Lazy image loading only applies to images in sections that can be collapsed so it won't apply on your "unnamed characters" or Jo Jo Wiki page as the headings there are wrapped in divs.

MobileFrontend intentionally doesn't support a lot of its formatting when sections are wrapped e.g. your <div class="diamond-is-unbreakable-tabs"> as we cannot reliably infer the structure of the page and initial visibility of the images. This is why section collapsing also does not work on those pages.