When a player, that is targeted by a phantom, changes dimension (nether for example), the targeting phantom keeps tracking the player as if it was in the overworld and can even damage/kill the player.

The conditions for this bug are:

-Phantom was targeting player before TP/entering dimension

-Coordinates before and after the TP are close (so that the phantom does not loose agro)

-The chunks where the phantom lives are loadded

Steps to reproduce:

In creative, create a nether portal at 8, 128, 8 (so the coordinates in the nether will be around 0~16, 64~128, 0~16)

If it is day, set to night or create a platform above the portal (so that phantoms will not burn)

Load the chunks (using /forceload command) around the portal (3x3 chunk is anough) 

Block the sides of the portal so only a player can walk into

Create a platform next to the portal with open top and spawn a few phantoms on it.

Set gamemode to survival and wait for phantoms to agro you

Enter the portal

Get damaged after a few second~minute by a phantom in the nether (the delay comes from phantom having to change direction to atack your new coordinates in the nether, which probably aren't at the same altitude/position).

With a spectator account that watches the overworld it will be even more visible.

This bug is very specific and almost never occurs in Vanilla without dimensions added by data packs (because the nether has a x8 coordinates scales, meaning the bug can only occur around ~0 ~0), but datapacks that add dimension will run into this bug quite often if they use a x1 coordinate scale with the overworld.

To my understanding, this bug comes from the agro of a mob not checking if it's target is in the same dimension, and probably affect other monsters too (only tested with phantom so far, since it is the mob that killed me in the nether while it was in the overworld)