My left Joy-Con was acting up (left stick was constantly losing the center), so I sent it to Nintendo's service center. Yesterday they said they fixed the problem. Just now I received the solution.
Today, Nintendo sent back the Joy-Con I sent them. And it'd been fixed! I don't know why they sent an extra pair of Joy-Cons yesterday, but I'm not complaining. So now I have four Joy-Cons.
Private instance for fsLeg where anything goes.