Blocks that are placed in downwards flowing water don't properly update water flow

To reproduce

1. Dig a 2x2x2 hole
2. Place 4 water source blocks on the top layer
3. Place 2 slabs diagonally on the bottom layer
4. Place 2 kelp in the remaining 2 spots on the bottom layer
   → Observe that the slabs do not become waterlogged after placing the second kelp.
5. Destroy a block adjacent to one of the kelp
   → The water now updates and waterlogs the slabs.