As the title says, grass that is under waterlogged blocks will not die, despite being underwater.

Steps to reproduce

1. Create or find a body of water
2. Place down a grass block under the water
3. Place a non-solid, waterloggable block on top of the grass block (seagrass, top slab, etc)
4. Turn up the random tick speed, or just wait

This does not work with mycelium, as mycelium does not appear to die underwater in general, which is odd in itself.

I was trying to get rid of some grass in my survival world, and couldn't due to this issue, and it took me spectating a copy of the world searching for any pockets of grass to realise the grass was just underwater.