This is caused by https://commons.wikimedia.org/w/load.php?debug=true&lang=en&modules=skins.minerva.content.styles&only=styles&skin=minerva (matches .content a > img)

.content a > img,
.content a > .lazy-image-placeholder,
.content noscript > img {
  max-width: 100% !important;
}

In T215271#4927813, @zhuyifei1999 wrote:

This is caused by https://commons.wikimedia.org/w/load.php?debug=true&lang=en&modules=skins.minerva.content.styles&only=styles&skin=minerva (matches .content a > img)

.content a > img,
.content a > .lazy-image-placeholder,
.content noscript > img {
  max-width: 100% !important;
}

This is not the problem. The problem is the template is being incorrectly identified as a page issue/ambox template. See my notes in the description. I'm not 100% sure how to fix this in the code level right now.. but there are certain short term template fixes that can be made.

.fmbox shouldn't be treated like a page issue because the class is not used for page issues - it is used for edit notices and system messages (see documentation of template here). Treating fmbox as a page issue also breaks things on enwiki e.g on (view logged out)
https://en.m.wikipedia.org/w/index.php?title=Barack_Obama&action=edit&mobileaction=toggle_view_mobile

A not useful "learn more" link is created and the padlock image removed.