Hello, this is a generalized bug regarding mob spawning and despawning. The mobs will despawn too quickly and mob spawning seems to have been nerfed comparing to other versions — I will be comparing it to mainline Minecraft, Java Edition. What I mean by nerfed is mobs will barely spawn at all when it comes to anything. The standard is below:

Steps to reproduce the bug (e.g., what you need to do in the game to get the bug to occur).

Go into a cave or anywhere else, even create a farm for mob spawning to get loot, such as a gunpowder farm or any farm for materials, the net result will be the same.

What you expect to happen when you follow those steps. 

Hostile mobs and passive mobs will be near non-existant compared to the same version on Java.

What actually happens when you follow those steps.

Mainly, you see 1-2 mobs around, a zombie, skeleton, maybe an axolotl here and there in the water underground. For mob farms, you see zero mobs.

Any further information about the bug, such as explaining any attachments you're including, when the bug first occurred, how you've tested the bug, messages displayed (e.g., error messages), and how the bug impacts gameplay. 

I believe this bug impacts the game because the game should be similar in comparison to every version of Minecraft, regardless of Bedrock or Java, even if coding is different, it is still possible to find the same result within the versions. It makes mining, trivial, and resources, diminishing and lack luster to spend hours waiting for night to fall to farm once for loot to sometimes never drop or rarely. I've tested the same mob farms both on Java and Bedrock, Java has the farm working correctly as it should be, Bedrock, on the hand, hasnmt had the farm produce anything at all. I 100% believe this is not an intented feature, more of a bug which hasn't been addressed for years.