That happens with all house carls you haven't chosen as a follower, even Lydia from Whiterun will do that, if you haven't asked her to follow you... and you can only have one such follower at a time. As long as they're just house-sitters they react that way.
If you've chosen them as your follower, then asked them to wait somewhere, they don't react that way, though. But, if you keep them waiting too long, they get tired of waiting and stop being your follower, reverting to being a house-sitter. They then go back to wearing their original armor.
Not reason for panic, though, since they're still carrying all the stuff you gave them, and all you have to do to access it, and have them wear that cool armor you gave them again, is ask them to follow you. If they don't put on the cool armor when you ask them to follow you, take something from them, or give them something new to carry, and they'll immediately put on the better armor.