The Bug:
When phantoms are hindered to fly up after their attack, their AI can guide them into flying under water such that they drown (see attached video). Possibly related to MC-125252

Steps to Reproduce:
I have created a roofed cage at sea surface (see attached image), where the phantom can try to attack, but can not harm the player. When it tries to fly away it is hindered by the roof. Attached please find a datapack with functions

1. to set up the structure,
2. to observe the bug
3. and to clean up again
   These include setting useful gamemodes, teleporting to proper positions, killing remaining phantoms etc.

1. Create new world with seed -2430049209890141612 in creative mode, which has a spawn point near oceans
2. Install the datapack
3. Start world
4. Fly to a position over deep ocean
5. Call function gulbaru:mc-126419_a_setup
6. Observe the bug: Repeatedly call function gulbaru:mc-126419_b_showcase_bug and observe how the phantom can not fly up as intended and ends up flying into water, where it will drown after some time.
7. Call function gulbaru:mc-126419_a_cleanup