Posting here, since I can not (yet?) post in the Bug Reports section.
I tried using many version of Forge for Minecraft 1.8.9, and the results seem to be the same regardless of what build is used(including the RB and 1723). When using Forge and creating a new world with Large Biomes selected, the world does not actually get generated using the Larger Biomes generator. The world gets created as a standard/normal world. Checking the generatorName key in level.dat does have a value of largeBiomes. It would seem that Forge is either not reading this key, or not using. I'm not really sure.
Here is launcher output from the moment the create button is clicked, until the world has generated and I can log out:
Here is entries from fml-client-latest during the same time:
I confirmed that this is an issue with Forge specifically by replicating the issue with and without mods, as well as testing the same seed out(in my test cases: 2013430788579336134) in a pure vanilla profile. If there is any other data I can possibly provide, let me know.