Adding runestone to armor does not increase AC

[GlitchCat]

When attaching a +1 Armor Potency Runestone to Explorers's Clothing which is equipped, I would expect my AC to recieve +1 to the bonus from armor.
However, this does not happen. Instead I have to manually add a fundamental rune to the Explorers's Clothing itself.
It also does nothing when attaching it to Leather armor

I assume this to be a bug, as manually adding a fundamental rune when the armor already has a rune attached seems to be an unnecessary step.