Problem:
Vindicators, Evokers, Illusioners and Pillagers won't attack each other when removed from the illager_friends.json Entity Type Tag list.
On the other hand; if you add a different mob to the list (for example, a zombie), the other illager mobs won't attack the zombie (which is the expected behaviour).

How to reproduce:
Create a datapack with the illager_friends.json Entity Type Tag list and the illager.json Entity Type Tag list. Since the illager_friends file by default just references the illager list we can edit either file. Add the "replace": true line and remove the above mentioned mobs from both or either list.
edit: I've attached a file with a minimal setup datapack, with both lists emptied out.
note1: even when adding and emptying the raiders.json Entity Type Tag list it still has the same result.

Summon 2 of the mobs, both with a unique tag for easy referencing. Then, use the /damage command to make 1 of the mobs damage the other, and watch how the damaged mob will not attack the other.

summon pillager ~-1 ~ ~ {Tags:["attacker"]}

summon vindicator ~-1 ~ ~ {HandItems:[{id:"iron_axe",Count:1b},{}],Tags:["victim"]}

damage @e[tag=victim,limit=1] 1 minecraft:mob_attack by @e[tag=attacker,limit=1]

note2: make sure to summon mobs with their respective weapons, otherwise they won't attack at all (Pillager=crossbow, Vindicator=axe, Illusioner=bow, Evoker=none).

Video to showcase problem:
https://youtu.be/j4c4UUdkUlo
note3: the video shows an incorrect editing of the datapack, the v3 version in the attachement is correct however and gives the same result as in the video.
note4: I realised that using the /damage command is way easier/convenient than using a snowball so I changed the description to accommodate that, the video shows the snowball method, both give the same result.