[Fern]

Make a custom mob and set their Stats to NOT scale, and then test it. The issue is that their stats end up bugging out and looking something like this no matter how much you increase them in the editor with mods:

 
Additionally, if you load the mob yourself, your stats will still scale.

[Neus]

This is not a bug, it's more outdated. That setting is specifically for bonus stats and additional stats per level, as well as max HP bonuses for bosses. As far as I'm aware, it doesn't affect the scaling that is done via stat_build. I'll probably update the tool at some point but for now that's how it works.