In the Civ VI New Frontier pass content you can play "Zombie defence mode", in which new Barbarian zombie units will spawn.
These zombies can spawn when a unit is killed by a zombie, when an undefended city is attacked by a zombie, and several turns after any unit has died there is a chance a zombie will spawn on the tile they died on.
But they seem to be spawning way faster than that in my current game. I also find that when a hostile zombie kills itself attacking a Friendly zombie (from the Twins Hero or Turn Undead), it spawns a fresh hostile zombie instantly, but if my Friendly zombie kills any unit, it does not spawn a zombie (Friendly or otherwise). I'm not sure if this is intended or a bug.
The zombies also have a "mutation strength" bonus to their Strength score, which I assume is to help them level up and stay a threat as the game progresses, but I am unsure how this is calculated, I couldn't find an explanation in the in-game Civilopedia. It's really running away in my current game, my 55 Strength units are facing +80 Mutation Strength zombies.
I understand that this is intended to be a difficult scenario, but it's weird that the zombies are acting in unexpected/unexplained ways.