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.
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.