Ducklings not hatching-could really use some help-Graphic picture

I posted approximately two months ago about ducklings failing to hatch: https://www.backyardchickens.com/t/896614/ducklings-died-before-hatching#post_13645270. Sadly I'm experiencing the same thing another time.

I have Ancona ducks eggs at 28 days. I've been watching closely-and haven't seen any movement what-so-ever. I took a couple of eggs out and candled them- they looked developed but I couldn't see any movement in the egg at all. So I did a float test-nothing. I decided to open an egg up to see what was going on.

I used tweezers to open the egg at the air cell. First impression was that the membrane looked a little dry on the outside. I was pretty sure the duckling was dead already so I proceeded to peel back the membrane. It didn't seem too tough. I could see feathers on the other side. I waited a bit-absolutely no movement from the duckling so I proceeded to open up the egg.

This is what I found. The duckling died sometime very recently. It hadn't turned around yet-it's bottom side was the part that I could see through the membrane. . What do you think is going on? The temperature has been right at 99.8-100.0 the whole time. Humidity was around 50% throughout incubation, bumped to 80% at lockdown (perhaps a tad high?). Judging from this pic-at approximately what day do you think it died? From what I recall this is exactly how my hatch in June looked.

I'm afraid the others have all died too as I'm not getting any movement at all in the float test. I can open another egg and photograph the membrane if that would help? I'm so frustrated-it seems everything is going great, and then boom-something happens and I lose them all.

I shipped some eggs in June and 10 of a dozen hatched...so I really don't think it's the eggs.