Fortress mobs not spawning on fortress intersection.

When I updated to new snapshot, the fortress mobs would no longer spawn in the 19 by 19 bounding box of the intersection of the fortress. This is for my wither skeleton farm. I had spawn-proofed the surrounding area to concentrate spawns in the 19 by 19 area; however, I got no mobs spawning. I removed all wither roses, but even if I didn't do that, wither skeletons should still be spawning I wonder if this is intentional, or a bug.