To reproduce
Dig a 2x2x2 hole
Place 4 water source blocks on the top layer
Place 2 slabs diagonally on the bottom layer
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.Destroy a block adjacent to one of the kelp
→ ✔ The water now updates and waterlogs the slabs.
it's same as MC-133814 - by placing second pickle you created second watersource and 2 water sources spreaded to waterloggable blocks like normal.