I think it’s because you search in:first. The post you get as a result is not 19h old, it seems to be from 2017.
But I understand your problem. If searching in:first there is no difference between sort by latest post and sort by latest topic.
https://meta.discourse.org/search?q=%40Discourse%20%23howto%20in%3Afirst%20order%3Alatest
Is an example here at meta.
The result shown last (Configuring Facebook login for Discourse) has newer latest posts than the first result