I noticed sometimes my damaged armor got repaired without going to any repair NPC after dying in PVE maps and re spawning with a spawn point.
Once it happened in lake doric when I got killed by champ bloodstone and re spawning straight away on a spawn point to finish of the event after dying, and then I noticed my damaged armor got repaired by the time I finished the event.
I did receive the message about the damaged armor though.
Sometimes I do have to repair it myself so not sure what triggered the auto repair.
Afaik leveling up and finishing events don't repair your armor.