The bug
Flooded/underwater rooms get a ceiling of water as if reaching an air surface, even directly below a solid block.
If the solid block is replaced with a waterlogged block the "water surface" effect is removed. But a solid block produces the water surface effect. This is intermittent, it does not effect all blocks in this way, however I have been unable to determine why, or find a work around this purely visual, but incredibly annoying bug for anyone building under water.
I think it has something to do with proximity to glass, extending 1 block away.
To reproduce
Build this structure and fill it with water
[media]Go inside
[media]
→ ❌ On the edges of the room's ceiling, you'll see the water surfaceBreak the middle block and replace it with glass
[media]
→ ❌ Now the water surface is visible everywhere
It would be expected that the water ceiling only renders from above, but not from below.
Related issues
is duplicated by
relates to
Attachments
Comments

I'm able to confirm this.
We have a ticket for the water surface being visible from above even if there are blocks above (MC-125546), but I don't think we have one for it being visible from underneath. At least I couldn't find any.
Can confirm in 20w51a.
Can confirm in 21w06a.

Can confirm in 1.17.

Can confirm in 1.17.1 Release Candidate 2.

Can confirm in 1.17.1.

Can confirm in 21w40a.

Can confirm
Can confirm in 1.18.2 and 22w16b.
Can confirm in 1.19.
Can confirm in 1.19.2.

Can confirm 1.20

if you fill a villager house with water a put block in hole at top it creates two lines with glitch, one line without

can confirm 1.20.1

Affects 1.20.3-rc1, and affects lava too.
[media]
Can confirm this for 1.20.4.

Affects 1.21.
I am also attaching the structure nbt file to load in your world using structure block 🙂