Slimes don't spawn at high light level
Steps to Reproduce:
Create a flat world with hard difficulty
Observed Results:
only passive mobs spawn in the daytime, slimes only appear in the nighttime.
Expected Results
Slimes should spawn in slime chunks regardless of time and frequently spawns in flat worlds.
I cannot reproduce this. Are you sure you don't have behavior packs or commands in use that could be affect spawning?