I did not do a lot of experimentatio to find all the circumstances in which this bug manifests but I can tell you my setup so that it can be reproduced:

A dispenser fires eggs into a 1x1 space that has a bottom slab in it. Underneath the slab there is a hopper. Somehow, almost all of the chickens end up under the slab and hopper. This is new to 1.13-pre5. I'm not sure whether it's the eggs or the baby chickens but i brlieve it's the dispensed eggs.