Looks like a bug.

This particular example is not using an infobox. with infobox we remove the float property on smaller screens so that it doesn't have this layout issue. it's even creating gigantic whitespace below the div.

.plainlinks.vatop < this div needs to have a float:none !Important; property based on the screen width. it should not float at all. it should be a block element. it fixes the issue.

This is an on-wiki template issue
https://www.mediawiki.org/w/index.php?title=Template%3AWikimedia_engineering_project_information%2Fcore&type=revision&diff=2509408&oldid=2509372 should fix it but I'm not sure how translating templates works and I'm not seeing it propagate to the right place.

Regardless this is out of scope for the reading web team.

@Nikerabbit - it looks like there is an issue with the template https://www.mediawiki.org/wiki/Template:Wikimedia_engineering_project_information
https://www.mediawiki.org/wiki/Template:Wikimedia_engineering_project_information/en is rendering for English rather than the up to date https://www.mediawiki.org/wiki/Template:Wikimedia_engineering_project_information - any idea why?

I recommend to read https://www.mediawiki.org/wiki/Help:Extension:Translate/Page_translation_example and and https://www.mediawiki.org/wiki/Help:Extension:Translate/Page_translation_administration where it is explained.

TL;DR: It is waiting for translation admin to approve the changes.

@Jdlrobson why does it need a table ? Also why is this not using infobox class ?

@TheDJ the rule is overly specific (table.infobox) to override default table css rules.
Now we have TemplateStyles on this wiki it would be better to fix it in the template itself though.

In T166456#3452724, @Jdlrobson wrote:

Thanks @Nikerabbit had no idea you could translate templates! Thanks for sharing the knowledge :)
... but it looks like https://m.mediawiki.org/wiki/Wikimedia_Discovery/Search is not running the latest version yet.. Guessing it has not been approved? But why does an English version need to be approved if English is the language it is written in?

The short answer is: because of the <translate> tags, the included templates must use the /en subpage, and all subpages are treated equally.

I don't seem to have translate rights or edit rights to the /en subpage. Shouldn't the change in the template be reflected in the /en subpage immediately? That's what's confusing me..

It isn't, because it is treated like any other translation subpage.