When slabs or stairs that have half=bottom are waterlogged, they do not display the water drip particle to signify that there is water above them. However, blocks directly below the waterlogged blocks do display the water drip animation.
MC-165877 Drips appear below a block underneath a waterlogged block, even if water cannot flow downwards from the waterlogged block
MC-127005 Bottom of waterlogged blocks shows water texture