Soo what if all the Plague anchients were knocked out cold, and it took them this long to resurface?
Or one of the anchients hit them so hard, they flew into the soup?
Maybe they've been healing and literally stewing in anger, ready to bust out of the soup at any moment!
Or perhaps Plaguebringer had seen what happened with Gaolers and Banescale and hid her first children away to keep them safe?
I'm gonna laugh if they were enemies of Veils, or Plaguebringer was trying to keep the two from fighting, but ADHD struck and one poked it's head out from the Wyrmwound only just now, spooking a local Mirror pack, or solo Mirror, or one looking for a new pack!
Maybe they were absorbing stuff from the soup, and only just now depleted or nearly depleted what was holding them there!