Placing the required materials (4 Soul Sands in a T shape, 3 Wither Skeleton Skulls on top) will not spawn the Wither.

I have tested with 1.13 vanilla and then without plugins. In vanilla, it spawns like it should.

Steps to reproduce:

1. Make a new world
2. /give @p minecraft:soul_sand 64
3. /give @p minecraft:wither_skeleton_skull 10
4. Place 4 Soul Sand in a T shape and place 3 skulls on top of the blocks - the placed blocks and heads stay there and the Wither will not spawn.

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