I have some chicken pens in a multiplayer world but sometimes when the chickens grow up, they age up outside of the pen and escape, this has happened many times, but I’ve never had this problem ever with versions before this.
MCPE-219366
Aging mobs outside of fences
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This happens with all mobs, but if you use the carpet bug to hop fences, baby chickens can get stuck underneath of them, regardless of baiting with seeds. This may be a related issue, if the chicks underneath the carpet age.