Yeap. Seems to happen with Quick Quote and Quick Reply, because for whatever reason, that function doesn't tell it what post the new post is in response to, so it sorta just "floats". Shows up in flat mode because it's in the thread still, and those are in chronological order, but because of it not being "attached" to another post, threaded won't display it.
My guess is it's a software bug.
Gravity seems weak until you look down.