I don't know. I usually look at late death (when not related to malpositions or hatching problems like ruptured yolk) as being attributed with incorrect humidity.
How long post lock down was this? Sorry, I'm not sure I've seen this before.
I took the eggs out of the auto turner and set them on their sides in the bottom of the incubator and then filled all the pans with water. I was able to get it up to about 70-65% RH with the temp at 100. Didn't open it or anything for 6 days until the one duck hatched.
Same thing happened to me, but it was due to the first hatched duck rolling the unhatched eggs. I set everything aright again, and still saw movement in the eggs around 11 pm the same evening that the first egg hatched. By morning, nothing. I'm assuming that after a few minutes of being rolled, they must have gotten enough fluid in their lungs that it compromised them enough that they faded and died overnight.
Sorry for your loss. We ended up with just one as well. I found a person about a half hour away selling ducklings about the same age, so I got a few to keep little Jr. company.