Glow squids spawn in cave waters, and zombies spawn there as well. Glow squids will die as they get out of water, and then zombies will pick up their ink sac, which means they can't despawn. Eventually, more and more persistent zombies will spawn underground, and it gets noticeably laggy.

In Java, zombies and their variants (husks, drowned, baby/villager) will not pick up glow ink sac, but this is not the case in Bedrock. Please fix soon!

As a side note, zombies shouldn't be able to pick up leads either, I often see persistent zombies with leads as wandering traders spawn and die in caves. And also, their Llamas don't despawn after the trader gets killed.