Usually when a player kills a mob with the player's armor on, the armor will be dropped usually undamaged. If the Thorns Enchantment is on the armor, sometimes damage will be reflected back to the player. At the time the player takes damage, the armor also takes damage due to the mechanics of how Thorns works. The main issue in this bug report is that mobs usually don't damage the armor in all circumstances (except for sunlight) and the armor takes damage from the Thorns Enchantment. I have attached a world with a spawner consisting of a "docile" skeleton riding on top of an undamaged diamond chestplate. When the skeleton spawns, the skeleton will wear the diamond chestplate as if the diamond chestplate was the player's diamond chestplate.