This works on creative or survival and I could not find a similar issue already reported. If this is a feature, please close this issue.

Bug:
Grass blocks exist and grow under water and under lava so long as the necessary lighting conditions are met. I would expect grass blocks to die under water/lava and I would expect dirt blocks that are under water/lava would not change to grass blocks when other grass blocks are nearby.

How to Reproduce (simple example, but not the only way to do this):
Find a 3x3 plane of grass blocks in a well lit surface area of any map. Dig 2 blocks deep in the center of the 3x3 plane. place 1 dirt block in the hole then put 1 lava or 1 water on top of the dirt. You can easily observe the dirt block change to a grass block after some time with the water. With the lava, wait a few game days and then remove the lava to see the grass block underneath.