Long title, but pretty sure this is a bug.
Whenever you equip a docent on an iron defender pet, it gains some stripes of a certain color. This works normally, and the dog will always have those stripes even after dismissing and re-summoning the defender.
However, once you pick up the Adamantine Plating enhancements for the iron defender, the skin of the defender actually changes. The docent in this case will revert the defender back to the original skin with the appropriate color stripes. After dismissing and re-summoning, the iron defender will change back to the Adamantine Plating skin.
I believe that counts as a bug, though I'm wondering if an iron defender with a docent is supposed to revert back to the default skin with stripes, or if it's supposed to stay with the Adamantine Plating skin and color in the highlight, similar to how the NPC iron defenders look. For example...
That's kind of what I expect it should look like with Adamantine Plating and Docent.