With the following setup:

the top faces of water and lava are still rendered, even though it's almost impossible to actually see them under normal conditions:

It probably isn't really worth wasting effort on drawing these faces in the first place, since they can only be seen under very specific circumstances. Also mojang has done this before with certain cases, such as removing it from the bottom of blocks with full bottom faces in 1.15 and the sides of blocks with full faces on said sides in 1.16.

probably better to leave these kinds of rendering issues to sodium