Could be this
https://www.merckvetmanual.com/poultry/avian-encephalomyelitis/overview-of-avian-encephalomyelitis
It's possible your flock is immune enough now that the embryos who hatch aren't infected, but there is still late embryo mortality.
https://www.merckvetmanual.com/poultry/avian-encephalomyelitis/overview-of-avian-encephalomyelitis
It's possible your flock is immune enough now that the embryos who hatch aren't infected, but there is still late embryo mortality.