Turtles that I have hatched and bred are despawning, as well as some of the eggs.
I play on peaceful mode so I know it's not a zombie getting to them.
I use a Samsung Galaxy S6.
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@Aspergian yes, i can confirm it is the case. Is it intended feature? Because it would be a shame if the answer is yes. I don;'t want to wait for 45 mins to make them hatch, when I can explore or build stuff instead.
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can confirm on 1.16.0.58 beta, with the new despawning spawning mechanics, baby turtles despawn unless named tag by the player, but adult turtles don't despawn, it's just the baby turtles bred or not it's always the baby turtle to despawn.u

This issue is now tracked at MCPE-70664.

Date: October 17 2020
PE version 1.16.20.54
I tested it in survival and creative. Once you feed the baby turtles sea grass they won't despawn anymore. It doesn't matter if they are adults. It also works on other mobs as well, once you feed them they won't despawn. hope it helps .-.
Edit: although I'm not sure if it would stay that way for a long time or not, but it works.

January 8th 2021
This bug is still apparent in version 1.16.201
Bug can be recreated within a survival or creative world, works very efficiently when entering and returning through a nether portal
Baby Sea Turtles seem to despawn if the player doesn't stay in the same area for too long (a few minutes or longer?)
But they do stick around if the player does stay in the same area, up until they grow into adult sea turtles.
Can you confirm if that's the case?