Oh that could be because I ended up removing that specific Config Value, since I needed to special-case a lot of Mods with the actual River Height, like Twilight Forest or TerraFirmaCraft.
I can fix the Issue ofcourse, but I would like to know what you need the River Height Config for? Is it a Mod or is it very custom Worldgen? Or a Dimension that needs it?
Edit: I just looked at the Code and at least for the Overworld the Config should work, so which Dimensions does this Problem exist for?