While trying to create a datapack that gives phantoms a neutral mob-like behavior, I discovered that certain phantom NBT tags cannot be edited with the /data entity command. I know this applies for changing name, size, and attributes (attackdamage and followrange specifically). All you need to do to reproduce this is target a phantom with a /data entity merge, most easily accomplished through /execute @e[type=minecraft:phantom] run data entity merge @s {your tags here}