There have been multiple bugs with water walls underwater showing up, it won't fill completely with water when I for example place slabs or trapdoors. One way i found out to stop the "waterwall" being created was to fill water with a bucket at a specific block underwater and then placing the slab or trapdoor on that excact spot, if not the wall will appear. Trapdoors will only create a waterwall if i place a block underneath. Glass will also glitch if it stands on a stair and i fill the water gap on the stair or just standing there at all. So in conclusion there is multiple bugs with waterflow underwater. Note, i am using a texture pack but i have included a photo without texture pack and the bug is still there. There is also a problem with surface level waterflow as seen on picture.