I only go completely dry if my bator stays above 25%, if it doesn't I add a wet spong which will usually hold it at 30% which is what I prefer. I did a late hatch last year and with the pellet stove running and it was too dry for me.
From what I understand, your darker eggs, because they have more pigmentation loose moisture slower than a lighter colored egg. The more "knowledgeable" can correct me if I am wrong, but it was something that came up in a discussion about BCM's and why they fair better with low humidity...
I'd also question the humidity levels for the first 17 days. Many late deaths (after lockdown) are usually low temps over an average or high humidity over the first 17 days not allowing the egg to loose enough moisture causing death at hatch time.