Lava that is under blocks has a hole in it's texture. While in the Nether I came across this issue. *See photo attached. This is the way the lava generated under blocks.

Steps to reproduce
-Place a 3x3 square of grass 1 block above the ground.
-Place lava under one of the corners of the square.
-The lava that spreads underneath the blocks will show a hole at a certain viewing angle.

The more blocks you place over the lava the LARGER the hole gets! See additional photo.