Mobs not spawning in Self-dug caves

So i dig a whole 16x16 wide with 3 blocks high cave starting at height 6, and many of them on top of eachother( with a purpose of "farming" them). its all dark, theres no other caves around that are not iluminated ( all other caves around are iluminated) . Absolutely no mobs spawn there at all, not regular zombies, not slimes ( which it was intended for ), nothing. i move away 40+ blocks, come back, still nothing.

Im playing on a realm. but i also have downloaded the map to try at offline survival and it doesnt work there either... i have "experimental gameplay" allowed so maybe it has something to do with it..