Items can not slide under upper half slabs in ice streams

Standard ice streams are a great way to move items around, however after setting up a series of ice streams that will move items along my chosen path I discovered that item entities can not pass under a upper half slab, which is the standard way of doing it. They seem to 'bounce back as if they are hitting some unseen force shield, though clearly they should be able to slide under.

I did mange to fix it using trap doors.