Monster try to spawn every tick in the distance of a sphere with radius of 128 block centered on player . Before cave and cliff update lighting up all nearby cave within 128 block is far easier when compared to after cave and cliff update. The update increased not just the size of underground cave but also the count , worse that in cave and cliff update there is now a submerged cave which either need to be cleared from water to monster spawn proof or put a lot of light source to prevent drowned from spawning. This is not much of a problem for monster farm since it can be built in the sky in the ocean where it is flat and 128 block up from the block nearby , but , this is a problem for slime farm . Slime spawn on slime chunk and swamp with very limited Y level , which means they cant be build in the sky where spawning algorithm won't try to spawn monster in underground caves. Because slime share the same mob cap with other hostile mobs , it means that slime cannot spawn once the hostile mob cap is reached. Please either fix slime spawning or make slime has it own spawning cap which is not shared with other hostile mob.