I built a mob farm in Bedrock edition, which doesn't work. It turned out that nothing spawned in the farm. It was built in a River biome and was completely isolated. As soon as I close off the roof in order to ensure the light level of 0, nothing spawned in the farm. It may work if the spawning platform is under direct sunlight or direct moonlight, but rarely.

Everywhere in the world has constant mob spawning. Slimes, Zombies, Skeletons, Strays, Spiders, Creepers, and passive mobs continuously and frequently spawn everywhere except in the farm. How can I fix the problem?

To reproduce the issue:

• Go straight up into the sky until being 128 blocks far from anywhere.
• Build a platform to stand on.
• Make a box of solid, full blocks (NOT stairs, slabs, glass or something like that) at least 8x8 (blocks) with the floor made out of either wood planks, stone bricks, cobblestone, sandstone, dirt, or stone.
• Light up the platform and around and on top of the box.
• Wait for about 20 minutes or 1 in-game day right next to the box or less than 2 chunks away from it (you have to stand at least 128 blocks far from any spawnable places).
• Break into the box to find out that it is completely empty.

Expected results: The box is supposed to have a few zombies, skeletons and some other mobs. However, it doesn't.

This is the process of my building the first farm, which is step-by-step described above:

https://www.youtube.com/watch?v=edSYAqVfTO4

This is the seed:

-7640521566208649721

This is the world download link:

https://hanoi01-my.sharepoint.com/:u:/g/personal/0132549206_hanoi_itrithuc_vn/EdWuLxoLav5PiA5afpPHXPMBybpiQXbbDAziLYbIOYPlCw?e=vwKm5V

These are the locations of the farms that didn't work:

41 216 -17 (above the house with a lot of cats next to it)

-272 195 124 (in the River biome in the middle of nowhere)