I had rest my phone without backing up newly purchased watch faces.  They can be restored by:

1) Open the galaxy store app on your phone

2) Click on 3 dots top right

3) Click on My page

4) Click on All

5) Click on Watch

You will see all your past purchases.

6) Click on the download arrow to download them to your watch again.

7) After downloading is finished just hit the play icon next to the face you want to use and it will make it active on your watch.

Hope this helps. Thomas