issues with mob spawning has been common in any world I create, flat, survival, creative, etc. Barely finding any enemies to kill or attack, in caves, in the overworld, Especially the cave part. I wondered in a dark cave for 4 hours and only found bats (yes I was playing on hard and not peaceful) and in home made mob spawners/platforms. However, regular cage mob spawners spawn at a good rate.

This is affecting me the most in Slime Chunks. Gathered slime chunk info via multiple websites for bedrock switch version, and slime spawn is incredibly low to none. 

To my direct situation, 5 total slime chunks next to each other with 5 platforms from Y Level 38 and down (lvl 38, 34, 30, 28, 24, etc) And I get maybe 1 small slime, and 1 medium slime every hour. 

If I was to guess, the issue with mob spawning is very low in Bedrock at this current moment.