Mobs no longer seem to spawn on elevated platforms.

Recently, I created a mob farm in my minecraft realm. It utilizes surface spawns and resides on top of a plains biome. It was built at y=110. However, not a single mob has spawned on it after I afk-ed 50 blocks under it. When choosing a different location, mobs still don't appear to be spawning. This had led me to believe that mobs are no longer spawning on elevated platforms, as on the ground, mobs are spawning just fine.

Steps to Reproduce:

1. Create a mob farm extremely similar to the 2 by 2 chunk design featured in this video: https://www.youtube.com/watch?v=fW1UwZeKhB8.
2. Change the time to night time so mobs can actually spawn.
3. Turn the farm on and afk 50 blocks under it.
4. Use a camera account if possible to see if anything spawns.
5. Nothing will spawn.