Wither body can no longer break liquids

The bug

Withers try to break almost all blocks they come in contact with; this includes water and lava.

However, as of 1.13 water and lava became both blocks and fluids, this causes the wither to still try and break the blocks, but the not the fluids, causing the water/lava to stay despite being broken by the wither (noticable via the particles).