Putting A Design on a Sheild Fixes the Sheild

I was playing in my UHC world when I wanted to design my shield. I created a banner and crafted that with my shield, and my partially used shield gained back all of its durability.

TO REPRODUCE:
1) Get a shield with partial durability
2) Get a banner
3) Craft the shield with the banner

You can see that the shield is now fully repaired.