Minecraft – Dark room Mob Spawner doesn’t work, even on Superflat at daytime

I know there are a lot of questions about mob Spawner, and I have read most of them and have tried to fulfill the following condition:

1. Lit up all caves and possible spawning location within 128 block: I'm in Creative with Superflat world type, so there is no caves, and I've tried to set the world to always daytime, so there are no other possible spawn location other than my dark room, meanwhile if I set the world to cycle day and night, mobs spawn normally on the ground at night, changing the world to survival gives the same result
2. Mobs won't spawn 24 block from the player, and will freeze if spawned more than 32 block from the player: My dark room floor is located 24 blocks away and the roof is 33 block away from ground
3. You're on peaceful: No, the world difficulty is set to hardest setting, even zombies can break wooden doors
4. Use solid block for mobs to be able to spawn: It made entirely of cobblestone and completely dark inside

My dark room mob spawner is the typical 20×20 block (outside frame), with cross shaped flowing water to the center of the room, so the inside volume is 18x18x8 blocks hollowed out. Already tried to separate the 8 block height to two and three floor, but still no result.

I've tried AFK in the collection point on the ground straight below the center of the room for an hour, there's no single mob dropped, and when I fly there to check, there is no single mob either. What could possibly wrong? Please advise.

UPDATE
Apparently my mob spawner works if I were far away from the bottom of the spawner, not sure how many block away, probably 24 blocks, my current guess is that you have to be at 24-32 blocks away horizontally AND vertically, not in sphere radius.

PS: I've done quite a bit of reading, but still kinda new to Minecraft, if possible please use noob friendly words or something that I can look up on Wiki. Also pardon my English I'm not native.