For example, a thread I like had recent posts today but I can't see the thread unless I search for the title, what is going on?

are they older threads that are being bumped by people other than op?

yeah basically, is that intentional?

Yeah. Most sections (if not all) have bump protection where if there's no activity for a few months and then someone tries to bump it, it won't go back to the top of the recent threads. At least I'm pretty sure that's how it works