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.
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Ok so the simplest way to reproduce it is the following: Place a dispenser in the air facing a slab and put a block on the other side of the slab. Most eggs will be dispensed under the dispenser instead of on the slab, how it used to be.
I got a chance to fire up the game today and notice that the baby chickens are falling out of the bottom side of the dispenser even though it's oriented towards the slab.