This issue affects all versions of minecraft bedrock, and it's a serious one.

As you know, zombies have the ability to pick up dropped items in their hand. And when they do that, they can no longer despawn.

The problem is that they can easily find some drops in the caves underground, cause it's not rare that a mob to dies there for several different reasons, such as contact with lava, fall damage, being out of water (for glow squids only) and more.

The items that zombies, zombie villagers, baby zombies, baby zombie villagers, husks and baby husks pick up the most are: rotten flesh, bones, arrows, chords, spyder eyes, gunpowder, glowing inc sac, slime balls.

Over time, zombies that picked up something in their hand tend to accumulate in the caves underneath your base, causing some serious lag.
This issue affects especially older minecraft worlds that people played for a relatively long period of time. (My world is one year old, I played it everyday for about 1 or 2 hours a day)

When I discovered this issue, I checked the caves underneath my base, and I found more than 50 of them in a ~3 chunks range!
They were really slowing down the performance of the game in areas where I spend a lot of in-game time, such as my base.

My suggested solution:
Zombies that hold these particular items in their hand (rotten flesh, bones, arrows, chords, spyder eyes, gunpowder, glowing inc sac, slime balls) will still despawn.