Not sure if this is a bug that has been reported, but you will get shapes just stopping, waiting to be fed into stacker (not sure if it does with others) but there is no space for them to be fed. So they just wait there.
Other shapes will pass underneath them, as seen in screenshot.
Not a big issue, but it does mean the further down stackers will not get enough, and when you have this happening four/five times it will impact feeds.
I would have thought (like I was sure was original?) that if there is no space to fit the shape circle, it would continue to move on and not wait.
It is almost feeling like the whole thing is timed, and thus ignoring "do I have space available?" - and maybe that is correct?