So, I was interested in seeing if I could get Iona to wear some more common clothes around the house. I pretty much just leave her there most of the time anyway, so there's no point in her being always dressed for battle.
I couldn't seem to get her to un-equip her standard armor, and she wouldn't put on regular clothing or lesser armor while it was equipped. So, after I finished leveling and perking up my pickpocketing, I stole the Steel Armor straight off her back. After that, I gave her some more suitable house attire (via "Follow me..." -> "I need to trade...") which she gladly put on. However, when I later left and returned to the house, I found her wandering around in her undergarments!
I checked her inventory, and found the clothes I'd given her were still there and her armor wasn't. So, she had no good reason not to be dressed in what had been handed to her. Later, I figured out this continues to happen even if I leave a full set of Legendary-quality Dragonscale Armor with her.
And, later on... I found out this is also the case with Lydia and Jordis. They'll all wear their clothes immediately after being given them. But if I leave them at home, I'll return to find them bare.
Is this a known bug? Is there any way around it without using the PC-only console?