Running version 1.7.10-2.1.0.1004-universal, with lots of other mods in a personal modpack (But nothing exotic or outside the common FTB mods) Seed 68589.

On disabling the polar chasm (id 114) entirely in the config and rebuilding the map, those areas instead spawned as lush river (id 116). A quick edit in biome painter solved my issue, but I'm assuming this is unintended behavior for worldgen.

I've been having a similar issue, same version. However, I have pigmen and ghasts spawning all over in the overworld (at least 100% of the time in a flat world, which are plains not lush rivers). I bet it's the same issue, because I've never seen a Polar Chasm biome in the nether, either. I'll disable that biome for now and see if that fixes things.

That sounds like the sub biome nonsense that was introduced by Vanilla. I thought the later versions using the BoP world type fixed it. See issue #334 It might need re-opening.