when attempting to spawn in a pillager, it will have the CanPickUpLoot data set as 1b/true, and cannot be changed. The same goes for pillager_spawn_eggs that are given to the player via commands with EntityTag data (even if it's empty).

Steps to Reproduce:

1. Summon a pillager:

   /summon minecraft:pillager
   

2. Observe it's CanPickUpLoot data:

   /data get entity @n[type=minecraft:pillager] CanPickUpLoot
   

   → Data is True

3. Try and modify it:
   /data modify entity @n[type=minecraft:pillager] CanPickUpLoot set value false
4. Repeat step 2

Expected Result:

The data would be changed to False.

Observed Behavior:

The data is not changed.