This may or may not be a bug. With slimes, it's hard to tell, since they spawn so infrequently. But, I cleared out an area of 50 chunks and set the random tick speed to 1000, and waited several real-time hours, but none spawned.

Details:

Seed: 3010043964128922788

In creative mode, I hollowed out a 50-chunk area from -96/35/-189 to -175/38/-34. I put glowstone under and above for the entire area (at Y-levels 34 and 39).

I lit up as much as I could around and above the volume I hollowed-out.

Then I set the random tick speed to 1000 to speed things up, and waited several hours of real-time. Not a single slime spawned.

NOTE: Slime DO spawn on the surface in swamp biomes (there's one nearby in this seed).