Upon summoning an ender dragon, using the component "NoAI:1b", you might realize, the ender dragons inner hitboxes aren't summoned at all, meaning you can't deal damage to it, projectiles will just phase through it, etc..

If you were to summon an ender dragon without this component, and add it afterwards, using the command "/data merge entity @e[type=minecraft:ender_dragon,limit=1] {NoAI:1}", you'd be able to interact with it, until you were to leave and re-enter the end, which makes the hitboxes disappear again.

How to replicate: 

Version 1:

1. Create a new world, enable commands.
2. Open chat and enter "/summon minecraft:ender_dragon ~ ~ ~ {NoAI:1b}".
3. Press F3+B to show entity hitboxes.
4. The dragons inner hitboxes haven't been summoned. 

Version 2:

1. Create a new world, enable commands.
2. Open chat and enter "/setblock ~ ~ ~ minecraft:end_portal".
3. Open chat again and enter "/data merge entity @e[type=minecraft:ender_dragon,limit=1] {NoAI:1}".
4. Press F3+B to show entity hitboxes.
5. Open chat and enter "/tp @s @e[type=ender_dragon,limit=1]".
6. Hitboxes are shown, as they should be.
7. Save and Quit to Title, and re-enter the world
8. The dragons inner hitboxes have disappeared.