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Comments
It is definitely a bug. Wither Skeleton spawns aren't supposed to be frequent. Even if it was just the soul sand valley, it would just be an excess of blazes, since blaze spawns are frequent. Wither Skeleton spawning should be infrequent spawns no matter which biome the fortress is in, so this is definitely a bug.
Even if it isn't a bug, something needs to be changed with the spawns because fortresses in soul sand valleys aren't even approachable.