Shulkers have an attribute modifier that increases their minecraft:generic.armor attribute by 20 when they are closed, but not if they have NoAI set to true.

How to reproduce:

1. Summon a shulker.
2. While the shulker is closed, execute the following command:

   /attribute @n[type=minecraft:shulker] minecraft:armor modifier value get minecraft:covered

   → The shulker has 20 armor points (unless you did it before the shulker opened for the first time, which is tracked at MC-272922).

3. Summon a shulker with no AI with the following command:

   /summon minecraft:shulker ~ ~ ~ {NoAI:1b}

4. Run the command from step 2 again (make sure to be closer to the new shulker).
   → The attribute modifier doesn't exist.

Expected result:

The closed shulker with no AI would have 20 armor points.

Observed result:

The closed shulker with no AI has no armor points.