The /team option prefix command does not save. After leaving the world you are working in on 18w21b, the prefix will disappear. Color didn't save also which was strange.

Steps to Reproduce

1. Join your world and change either the color of a team or add a prefix.
2. Leave the world or stop a multiplayer server.
3. Rejoin or reload the server and join.
4. Boom. Your settings never saved and are gone.
   Attached are some pictures to help understand the context.

The orders of the pictures are

1. Typing in the command to add a prefix.
2. Typing in hi with the prefix
3. Leaving the game.
4. Typing in hi without the prefix when rejoining.