I had a very low-durability shield and when I decided to craft it into a banner, the shield got repaired. I tried to test it on mobs and it really got repaired as the shield got damaged and it didn't go back to its previous durability.

Code analysis: https://bugs.mojang.com/browse/MC-87048?focusedCommentId=299652&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-299652