I believe this is because when they made the “change to the “spawn explosion breaking obsidian” they just changed the wither’s block breaking attack, which is also what the dash is just horizontally moved with the wither, because the block breaking attack appears to happen upon initial spawn even when the wither isn’t damaged.
Before the 1.16 update, the water would not be moved, but the block could be moved in and out of the water without the water being destroyed. They did this to remove zerotick farms and called it “parity” even though on the Java edition waterlogged blocks do not lose their waterlogged state when moved.
Pay attention to cobble walls at wither’s “feet”
This allows to get bedrock in survival
This is not a bug. When mobs spawn, they spawn in the same block, most mobs in groups of 2-4. This should be marked as works as intended.