When an entity is in another dimension, teleporting it to yourself while you are also in another dimension different from the entity. The entity will dissappear.
Example: A chicken is in the nether and you the player is in the overworld. Using the teleport command to teleport the chicken to you will cause it to dissappear.
Video Example 1 : Summoned 2 chickens, Chicken far away from the portal will dissapear when teleported.
Video Example 2 : Summoned only 1 chicken.
Video Example 3: Summoned only 1 chicken with ticking area.
Even though the chicken doesn't teleport the command still registers the entity being teleported to you the player.
This also happens with custom entity with persistent and tick_world with never despawn as true. They would still dissappear upon teleport.
Also, happens when using the teleport method from gametest.
MCPE-127414 Teleporting non-player entities to a different dimension despawns them