Apologies for double posting again, but I think I have pinpointed the problem with MythicMobs.
-Adding the vanilla parent biomes (the ones which I put in "ReplaceToBiomeName" in my custom biomes) in the MM config doesn't seem to work at all.
-The only instance that MM biome-specific spawning worked, was when I added the name of the custom biome itself in its config.
Now, the problem is that, strangely, it only works with custom biomes which do NOT have a value in the "ReplaceToBiomeName". For the ones that have, neither their custom biome name works, nor its "parent" vanilla biome. (And unfortunately I need "ReplaceToBiomeName" for saplings and climate).
Any idea why this happens anyone? I am suspecting that it's the same problem that makes custom mob spawning reset to default when you try to change it in a vanilla biome.