SSY090106

Is there a spawn-able block below the scaffolding?

Did you interact with the brown mooshrooms, ie feed them?

Are the mobs on a chunk border? If so, this bug is MCPE-21416

(world download above)
I made a world with the same seed as my survival world and cleared out the chunk. 3 commands in command blocks were used:
/kill @e[type=!slime],
/execute @e[type=slime] ~ ~ ~ setblock ~ ~-1 ~ glowstone,
/kill @e[type=slime].
With these commands, the surface and cave cap would be completely empty. However, no slimes spawned in the slime chunk hollowed out to y 30 .
That same slime chunk spawned slimes in a super flat world. I'm not sure if this bug is platform specific but I am experiencing this on iOS.