I’ve just discovered that on at least one of my sites, if a dead URL is entered without a trailing slash, it correctly redirects to my 404 page. However, if there is a trailing slash in the URL, it leads to a ‘text-only’ version of the 404 page. It also seems to be leading to the mobile version of the 404 page, even on a desktop. There’s a giant hamburger menu icon showing.
This works as expected -
This gets a text-only version -