I have a Buff Cochin Hen named Mattie who is on day 19 of sitting on Ameracauna eggs, I didn't know that she was broody, so I just kept collecting her eggs. She gets off them, rolls the eggs around from one end of her broody-pen and then she will tuck them under her. She was off one on Sunday and didn't pick it up. I checked the egg, and there was a chick inside. What do they usually do. I have only had one other hen lay eggs, and she was a Bantam Cochin, I never saw her off her eggs, I only saw her off when her two chicks hatched, so I am baffled.
I would keep the egg under her and give it a chance to hatch. My broody got off her eggs multiple times and let the eggs get cold before getting back on and I still got one to hatch (only had five eggs under her). Also, hens can actually tell when an egg/chick has something wrong with it and will sometimes push that egg out of the nest ... my hen did that too and she was right ... it was a bum egg. However, since you know there is a chick in there I'd try to get it back under her and see if she'll keep sitting on it to at least Day 23.