http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/2524489-realistic-terrain-generation-rtg-realistic-biomes?comment=3653

I noticed this before, some time ago, (I believe in 1.7.10,) that flat bedrock wasn't working in mod-added biomes. @Zeno410 said something about it that explained why it wasn't working, but I can't recall the details of that conversation. I thought that it had been resolved, but maybe it wasn't.

This was before porting 1.7.10 to newer versions of Minecraft.

I've just tested this with RTG on its own, and with BOP, and everything seems to be working fine in both vanilla and modded biomes. I suspect that the user had another mod installed that also affected bedrock generation.