I play on a world that has this mod and Biomes O Plenty installed and it's extremely rare to have the regular skeletons convert into wither skeletons as they are supposed to. It's because most areas are not Hell biome in the nether anymore. BoP adds five new nether biomes: https://biomesoplenty.fandom.com/wiki/Biomes_List which ends up making the regular Hell biome fairly rare to find, much less with a fortress in it.

It's a small issue but one I was hoping could be fixed if you have the time for it. Oh, and I'm using the 1.12.2 version if that matters.