One or two were old, trying to store honey. The rest were new, after the Queen awoke.
It was precisely this last event that triggered the softlock. I have no programming experience, but I think it gets stuck in a loop trying to spawn a new unit in an occupied space.
thank you for the detailed info! one other thing that has me curious is the 2 blue ants in the top right. i doubt it has anything to do with the softlock, but i want to know if their positions were offset like that the entire time