I would like to up this bug and give some data on the bug's behaviour:
When you apply Stoneskin Infusion on the character that wears the legendary armor (stance doesn't matter in this case), the look out doesn't change.
When you mount/dismount or hide/unhide an armor piece the infusion works appears on the whole character, including armor and skin.
When you draw your weapons (so you change idle stance to battle stance) the appearance from the armor disappears and only your character's skin remain stone.
You can reset this by mount/dismount or hide/unhide an armor pience, otherwise it stays that way.
My suggestion is - since legendary armors works a bit more complicated I can guess - that make Deldrimor Stoneskin Infusin work ONLY ON SKIN, when you wear a piece or multiple piece of legendary armor. Or just fix the bug alltogether if that is an option.