This glitch does not only happen in Savanna biomes, and it does not only happen with grass. In Forest biomes, sometimes trees will spawn over lakes like in the picture.
I have been struggling with a similar issue in my base except the area was much smaller. It was a hole in a wall formed by a small cave that I left open until I could fill in the wall. I made sure there was a torch in the middle but mobs kept spawning. It couldn't have been a mob spawner because multiple mobs (skeletons, creepers, and endermen) spawned.
Samuel Voltz
MC-105605
I want to say it does happen sometimes in 1.10.2, but doesn't occur the same way.
Still happens in 1.10.2, I even took a photo before finding it was already reported.
This glitch does not only happen in Savanna biomes, and it does not only happen with grass. In Forest biomes, sometimes trees will spawn over lakes like in the picture.
The glitch still works in 1.13 proper. I've only gotten result #2 in my expirience so far.
I have been struggling with a similar issue in my base except the area was much smaller. It was a hole in a wall formed by a small cave that I left open until I could fill in the wall. I made sure there was a torch in the middle but mobs kept spawning. It couldn't have been a mob spawner because multiple mobs (skeletons, creepers, and endermen) spawned.