For me I had a similar problem, but they didn't lose it, they just unequiped it.
So if I wanted them to keep the fancy new equipment on, I had to keep them as followers and not send them home. Or I had to make them a follower, take one item back, give it to them again, and then they'd equip all their fancy new threads.
It's quite a wierd setup regardless. Not really sure what went wrong with the programming here. If we give better gear to them, I don't want them to default to their default gear, ever (unless I tell them to).