Emmett Barrett
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I'm pretty sure the reason why this happens is somewhat pertaining to the fact that you cannot have any blocks placed underneath the arms in the T formation of the soul sand, and the only "blocks" technically allowed there are Cave Air and Air. The reason they aren't spawning below Y = 0 probably has to do with the fact that all the air below Y = 0 is still considered Void Air, thus not allowing the Wither to be invoked.
Just Minecraft logic tbh, not even sure if this is a bug more than an annoying feature they need to change. It's so portals between dimensions can be linked up more cleanly.