When using the NoAI tag, all hitboxes are grouped together, and the dragon cannot take damage. It also doesn't show Boss Bar (MC-176921)
Image #1: Bugged Hitboxes (Appears in both versions)
Image #2: "Normal" Hitbox (Top Hitbox doesn't show in 1.15.2)
Environment
Ubuntu 19.10
Duplicate of MC-74114.