<span class="mw-page-title-main"> is added by Parser::formatPageTitle.

include/skins/Skin.php

if ( !$titleMsg->isDisabled() ) {
        $htmlTitle = $titleMsg->parse();
} else {
        $htmlTitle = $out->getPageTitle();
}

So getPageTitle here is adding the mw-page-title-main class, because that's the title formatting at the parser level. The mainpage override is a skin option, and the skin does not add this class. Still trying to figure out where the <p> is coming from. Probably from msg->parse() (update: switching the msg to use <div>, does remove the p elements....)

Can't seem to reproduce this locally yet....

Suspect this is due to DiscussionTools, which I do not have installed locally. The html also has DT related comments inserted, which I think is new.

<header class="mw-body-header vector-page-titlebar">
				
				    <h1 id="firstHeading" class="firstHeading mw-first-heading"><p><span>MediaWiki is a collaboration and documentation platform brought to you by a vibrant community</span>
</p><!--__DTHASLEDECONTENT__--><!--__DTEMPTYTALKPAGE__--></h1>
				
				
				        <div class="mw-indicators">
        </div>
</header>

Alternate possibility might be the header rework ? Tagging @matmarex they'll likely know

Oops, it's another case of T291833.

Change 880571 had a related patch set uploaded (by Bartosz Dziewoński; author: Bartosz Dziewoński):

[mediawiki/core@master] Set interface flag for the main page title message

https://gerrit.wikimedia.org/r/880571

Change 880571 merged by jenkins-bot:

[mediawiki/core@master] Set interface flag for the main page title message

https://gerrit.wikimedia.org/r/880571