Shulkers are not affected by the Invisibility effect

I was using an invisibility splash potion on a shulker and for some reason the shulker's shell didn't become invisible. The head of the shulker should be the only part of the shulker that should be visible

To reproduce
1. Spawn shulker
2. Splash shulker with invisibility potion

Note: In the attachment, I've used the /effect command. Notice the chat above the screen, it says "Given Invisibility *0 to Shulker for 30 seconds" but the shell of the shulker is visible. I have no repeating command blocks in the world.