Well, I don’t have a 401, a 701 instead with a totally different rear light condition, but I have an opinion! And it’s an internet forum, so let me tell it to you: this play in the rear light fastening is a good thing. I have seen it before in other motorcycles, especially in mounting turn signals. You can take bumps, your pillion passenger can kick it accidentally, and no harm done. Kind of a soft mounting, designed intentionally to withstand abuse.