NCF – Extended list of Neutral creatures.

So it’s most likely my mistake and not related to the mod?

I can't say for sure. Try following RedHeavenHero's advice and generate a map in the regular version of Heroes, without using NCF.
Also, there is something called IRMG. It's an alternative map generator; it works without Heroes altogether. Although the maps are slightly different, in my humble opinion, they are worse. But at least bugs occur less frequently on them.