I used https://www.chunkbase.com/apps/slime-finder to find a slime chunk, I then cleared out a 16x16 area all the way down to y=6 and no slimes seemed to spawn.  I had layers with 3 blocks of air between each one up to y = 37 and no spawns.  So I copied the world, went on create and used /fill to fill everything within 63 blocks away in glowstone and still no spawns.  I opened a different seed and used /fill to do the same thing at the same coordinates and slimes started to spawn.  The world I couldn't get slimes to spawn is 654047387 with coordinates (688, 6, -688) - (703, 40, -673).  I am in some snow biome and I just tested this in the 1.16 beta and still now slimes spawn there