I'm 3/4 for my English Orpingtons that went into lockdown. 3 are out and gorgeous. The 4th I was concerned about it, since before lockdown it had no air cell visible, but was clearly alive and moving. After the 3 chicks hatched and dried, I took the last egg out to give it a quick candle and the chick had died. It was a full term chick, but still had the entire yolk attached and I think without the air cell, she just couldn't get thru the external pip to get air.
I still have 8 Blue Marans in my other incubator and I'm going to set 8 more Blue/ Splash English Orpingtons tomorrow.
Sorry for the little one you lost. Would love to see the babies. Post pics.