That's interesting. Sprayed several cars over the years and always used cellulose. My Vitesse I sprayed a few years ago took a year before you couldn't leave a fingernail mark in the paint, a nightmare really, especially after all the restoration and new panels up to that point. I don't think cellulose is the same chemical mix as it used to be. My Dad was a paint sprayer, using cellulose through the 60's and 70,s until it all went 2k and never used cellulose again. I would help him as a boy, and he would spray the car in cellulose one day unmask the following, and then leave it out in the open with no issues.
The other problem I have found with cellulose is that it shrinks, and can develops hairline cracks along seams.
I'll have to look into the non Iso paints, got a spitfire to paint at some point and don't want the nightmare I had with the Vitesse.