Not a bug, topics with > 20 replies do not have all content loaded in the DOM – we only load the visible posts, and unload the rest – so CTRL+F in that case would not find what you want to find. Please see many other topics on this here.
On short topics, all 20 posts fit in the DOM, so we don’t override search.
edit: ah, I see you are trying a two letter in-topic search. That’s a different issue, re-opening.