Ok, it took a bit to figure it out. First the short answer:

Mod 75% more women breaks it, if a thrall was spawned with that mod active.

My testing was initially flawed. I had lots of thralls that I’d captured or created while it was active and with the mod in place, got my 0% growth issue, then I went and removed all mods and checked the thralls again and they still showed 0%, so I thought it wasn’t mod related. So I installed a second copy of Conan on a second computer, to make everything 100% clean with no mods ever touching the game. I was worried that somehow a mod corrupted a game file, even though I’d verified them all.

Well, clean install worked great. So I started adding in mods and spawning thralls after each mod and that gave me the answer.

I’ll post in the mod comments for the creator to look at.

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