So, if I get this right, it seems there’s a couple of different bugs here.
In my example, it was also a bookmarked post and it seems you got the same result when you bookmarked a post: not finding it with in:bookmarks in:title (but fine with each filter individually). Let’s call this bug1.
But it seems you found a different bug: a bookmarked topic not showing with just in:bookmarks — I think that looks even more odd? I suppose one could try to make a case (not one that I would agree with) that a bookmarked post maybe shouldn’t return for a search with the topic title and in:bookmarks since the title is not on the bookmarked post itself (but on the topic). But not returning a match for the title of a bookmarked topic and in:bookmarks seems something is amiss too, specially as it works for bookmarked posts.
While I’m here, if someone looks takes a look the bookmarks code for this, maybe have a glance at another bookmarks related issue I recently reported which is why I have been poking at the bookmarks feature.