I got this bug too, where my wither skeleton pads stopped working. I used the same seed and generated the world in the snapshot, went to where the fortress was, and found a fortress next to where the old one was, but no fortress in the area where the pre-13w18a fortress was.
I got this bug too, where my wither skeleton pads stopped working. I used the same seed and generated the world in the snapshot, went to where the fortress was, and found a fortress next to where the old one was, but no fortress in the area where the pre-13w18a fortress was.