Switching to Peaceful Doesn't Remove All Hostile Mobs from the World

Switching the World to peaceful only removes hostile mobs in the chunks loaded around the player.

I was expecting all of the hostile mobs to be removed from the world (apart from those exempt, such as the Ender Dragon or shulkers)

Steps to Reproduce:

• Begin in a world with the diffuclty set to easy or higher
• Enclose a hostile mob in a walled area
• Teleport away (/tp ~1000 ~ ~)
• Switch the game difficulty to Peaceful
• Switch back to Easy or higher
• Teleport back to the original location (_/tp ~-1000 ~ ~)
• Notice that the hostile mob originally spawned is still present