When using Biomes O' Plenty witches do not spawn in witch huts unless decorationHacks is set to false. I believe this is due to RCTerrainGenEventHandler.onInitMapGen initializing a vanilla MapGenScatteredFeature object.

BoP extends this class including the isSwampHut function to look for an instance of BoP's own structure start rather than vanillas. However, the vanilla MapGenScatteredFeature.isSwampHut ends up being called. I'm not entirely sure what a proper fix would be, but I think altering event.setNewGen to initialize a new class of the type event.getNewGen() in the event handler might do it?

Thanks!