Duck egg halfway hatched, dead, on day 21

[Oob Child]

Update: when I came to check on her tonight, the polished egg was out of the nesting box, on the floor and cold. I candled it and I thought I saw some movement? But I'm not sure. I read online that she might have known the embryo inside that egg died and instinctively pushed it out. But I dont know how she could even do that. The nesting box is one of those black plastic crater with pretty tall 'walls', enough for her to get out but not the eggs. I dont understand how she might have taken it out. The egg did not crack further though, interestingly enough. I placed it back besides her and she seemed to take it back under her. Does anyone know what I should do know? Assume the egg is dead and take it out so I dont risk ruining the other ones?

I have no idea. Maybe Nabiki can help. I don't know anyone who knows about hatching without an incubator.. using chickens.
@Nabiki

 

[Nabiki]

Update: when I came to check on her tonight, the polished egg was out of the nesting box, on the floor and cold. I candled it and I thought I saw some movement? But I'm not sure. I read online that she might have known the embryo inside that egg died and instinctively pushed it out. But I dont know how she could even do that. The nesting box is one of those black plastic crater with pretty tall 'walls', enough for her to get out but not the eggs. I dont understand how she might have taken it out. The egg did not crack further though, interestingly enough. I placed it back besides her and she seemed to take it back under her. Does anyone know what I should do know? Assume the egg is dead and take it out so I dont risk ruining the other ones?

I would check it in a couple days to see if it's still developing or if it's dead. Mark it.