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

The achievements obtained recently do not seem to have this problem, only the achievements obtained long ago do. Maybe the resolution was to prevent duplication of achievements obtained but did not fix the already duplicated old achievements.

Yes

(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.