Well, I don't want the zombies to spawn with any states applied. It's not the cause since it happens with and without. It also haopens without VisuStella, so it's not that either.

I looked at your screenshots and I see you do have an enemies array setup for the map.

Highlighted is the default enemies which I see you have configured. On my end I have also setup region enemies without modifying the code:

Enemies do spawn normally in which I get two slimes.
The code error comes from using the spawn enemy function which has a different line up on my end.
Are you using the latest version of the plugin? I did a series of rapid updates recently.