I wouldn’t be so quick to blame the high spike in the humidity. It’s more probable that because of the relatively high humidity throughout the first 18 days, that there was just a lot of fluid in the eggs that didn’t get a chance to evaporate out. So when the chicks did their internal pip, they drowned. This would have happened with or without the spike.
Plus, that spike happens with each chick that busts out of its shell all sopping wet. It’s normal and not a problem.
The accepted advice is to incubate at 50% humidity, but in my experience, that is incorrect. I had a lot of “pip and drowns” when I was new and followed that advice. I especially noticed the pattern when I was incubating out in my garage, and as the season (spring to summer) progressed and my garage got more and more humid, the drown toll increased. So I stopped adding water during the first 18 days, and the problem immediately got solved.
I would venture that your survivors were simply lucky that they didn’t drown too. Not because they were first, but because they just luckily didn’t have too much fluid. And the later ones didn’t die because they were late, but rather they were “late” because they drowned (right on time).
I’m sorry for your bad experience. I know the frustration of seeing perfectly good chicks dead in their shells. The successes are what keeps us going, huh?