Hey @BrendonKoz, that is weird. I also did a full text search without any hint. But this appears not only on localhost here are links to two processwire powered sites on complete different webhosting companies and there is always the chrome lighthouse warning:
https://www.piluweri.de/
https://www.mobilo.net/
And this is a Craft CMS powered site on the same webhosting like the mobilo website and there is no Expect-CT header set:
https://www.jutt-immobilien.de/
I also investigated the htaccess default file from pw without any hint. Do you have any idea what causes this warning? or anyone else?