Saw it again earlier, and again Wandering Zombie was involved. Funny thing was that this time i had a total of 6 zombies which I don't think I've seen before, usually only when I have exactly 5. @Sokolov I guess you have a lot on your plate right now but would be nice to see this thing finally get fixed. If there is any kind of testing I can do that might help let me know.
Could be but this issue has been around for as long as i can remember. As far as i can tell it is triggered by 'respawning' unit like Wandering Zombie or Coragh dying when you have exactly 5 zombies. You still appear to get the proc but no swarms until a 6th zombie is deployed.
here is a screenshot i took recently (after patch) showing the zombie apocalypse did not trigger (no swarmed festering corpse was created next to my risen moga that killed the enemy brightmoon lich with the risen moga's basic attack.).
