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).
I may have created this ticket twice. If so, I apologize, and you can close the second one as a duplicate of the first.