Make a 21x21 slab that is atleast 5 blocks away from the ground.
Put water on it. The water will spawn underneath the slab
Expected: water will spawn in the slab not underneath it
Video: https://youtu.be/b6Xdtp_NDKg
MC-279007
WATER GOING THROUGH SLAB BUG
Resolved
Invalid
0
Slabs are waterloggable, so using water on them places the water in them. Since your slabs are in the top half, the water will be "below" the slab.
See MC-173730 for shift-placing not working.