It seems that Discourse only uses the Discourse-powered in-topic search if the topic passes the 20 posts mark. Before that, the browser-based search is triggered via Ctrl + F.
If the browser-based search is used, the search phrase is found when the post which contains the search phrase is somewhere near the current scrolling position.
For example in this topic, the search term
There isn’t much to learn in RSS is found when searching from beginning, but not from end (but from post 12 it’s found again).