The bug

When attempting to modify a phantom's attributes with the /data command, changes do not take place. Other functions of the /data command work fine, such as changing its current health, size, etc. but attributes such as attack damage and follow range are not affected.

How to reproduce

1. Summon a phantom
2. Run the command (The base modifier can be any number. It makes no difference): 

   /data merge entity @e[type=minecraft:phantom,limit=1] {Attributes:[{Name:generic.attackDamage,Base:0}]}
   

3. The server will say the mob has been modified, but checking using the following command reveals that no change has occurred:

   /data get entity @e[type=minecraft:phantom,limit=1]
   

The commands used above function correctly for all other mobs I have tested.

Code analysis

From 18w31a's source:

Method ang.l() will directly set the phantom's generic.attackDamage to a hard-coded 6, plus the value from their Size tag, which overrides whatever Base was previously specified. This method is called when the entity has its data set from NBT (whether it's via command or via chunk loading).