The bug
When a fluid source is surrounded on all four sides and from above by either full solid blocks or other fluid sources, it does not appear as a full cube. This is an issue for several reasons:
This constitutes a parity issue, as this behaviour is present in Bedrock Edition, and is valid due to 1.18.2's changes made to fluid rendering.
In certain cases such as ice above water, that water does not snap upwards to the ice, which causes weird-looking inclines at block edges.
In Spectator mode, the tops of underground fluid bodies are always visible, even though the other five sides never are, which makes visibility needlessly difficult.
How to reproduce
Create a 3x3x3 hollow cube of glass (optionally have one side be a separate block that allows pixels to be counted)
Place water or lava in the center of the cube, and then close it in the cube
Expected results
The fluid inside the cube would appear as a full, 16x16x16-pixel cube.
Actual results
The fluid inside instead appears to be a roughly 16x14x16 cuboid.
Water renders correctly if the block above happens to be waterlogged:
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