Creator for all official mash-up packs appears as "Unknown" in the new template screen

Steps to Reproduce:

1. Own any of the official mash-up packs by Minecraft
2. Switch to the new Play screen
3. Click on the "Create from template" button
4. Click on the "Owned by me" button and locate the mash-up pack
5. Look at the "By (creator)" text

Observed Results:
The text says "By Unknown".

Expected Results:
The text should say "By Minecraft".

Screenshots: