While playing survival, I placed a Diamond Helmet into an enchanting table with 16 active bookcases and retrieved a level 3 enchantment with both Protection III and Projectile Protection IV. My understanding of Minecraft enchantments states that these conflict, and this shouldn't have happened.

If it's of any use, the guaranteed enchantment was just Protection III and I was level 32 when enchanting.

As enchanting is random by design I cannot provide reproduction steps.