Yes, the <$hn> tag should work in the section prefix/suffix and anywhere outside the header/footer; the current behaviour is buggy and the placeholder is only compiling properly in those location if it is also used in the title or title prefix/suffix (which is why it works in @Vincent_Vincent’s example above). As part of the bug, seemingly, the unresolved tag in the prefix ends up on its own line, whereas normally the section prefix would not be without manually inserting a carriage return.
Given the bug, the way to do this for now is to enter the placeholder in the title prefix and tick the option there for “Insert title as run-in head”. That will put it on the same line as your following text.