A recent change[1] has caused a regression of T165115: action=mobileview sections delivering a div of class "mw-parser-output" without a matching closing div, causing the same consequences for the mobile clients (see T186927#3959632).
It appears the code for handling the new 'unwrap' post-cache transform (which is supposed to prevent this from happening, see https://gerrit.wikimedia.org/r/#/c/399910/) is flawed in that it expects that the closing </div> for the mw-parser-output div will be at the end of the document. In fact, debug text may follow.
[1] rMWd511626236f9: Add 'unwrap' ParserOutput post-cache transform https://github.com/wikimedia/mediawiki/commit/d511626236f9eeeb6a55c97c3c0d74d30150dc4f