This is probably related to MC-125252
When a Phantom is summoned from a command block with specified NBT data, the phantom will fly straight at the direction 45.0 (Southwest) unless you are able to "disturb" the phantom and then which he will attack you if you are in survival/adventure mode.

When the Phantom is summoned without NBT tags, the Phantom will nose into the ground, like normal (due to MC125252)

How to Reproduce:
in a command block:

 /summon phantom ~ ~1 ~ {CustomName:"{\"text\":\"hawk\"}",Invulnerable:1}

Compare with a phantom without the NBT data.