This is a barely a duplication of 6872 but not exactly.

6872 shows what happens to JUST WITHERS but not explane how and why.

When making a wither boss NORTH TO SOUTH it deletes the soul sand and heads and u have a wither.

When making it EAST TO WEST, it leaves blocks behind.

ALSO when trying to make 4 Staris into corner staris in a 2x2 area. when you're trying to do it faceing north or south it is much harder than faceing east and west.