When creating the pattern for summoning a wither (but placing the skulls first, so the wither does not spawn and you can use it as a decoration) and then placing a wither skull nearby, the wither will be spawned. For more detail, watch the attached video.
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Some really interesting behaviour - the block update seems to have to come from a specific direction. It happened for me in all dimensions. Note that I didn't do it exactly like in the video, instead in an open space where I first built support for the heads that isn't soul sand, then place the heads, remove the support blocks and then place the soul sand.
Confirmed in 1.15.2! Hence this may be a duplicate of MC-74249.
Same thing happens with snow golems and iron golems. I created a new issue for those (MC-233107).
Removed the part about updating blocks that would presumably cause the wither to spawn, since it is incorrect. What’s spawning the wither in that case is MC-74249.
I tested this in 20w19a, and it doesn't work in the end dimension for some reason. However it works in the nether and the overworld. (Works in complete vanilla, no mods or datapacks)