When a controller dies, one handed movement kicks in as it should; this works perfectly. But if the hand that is displaying the One Handed UI dies, it will be locked to the HMD.

If there's no controller to provide input at all, the One Handed UI should vanish for that controller instead of being locked to the HMD. Alternatively, change the default disabled position to just be out of view of the HMD. It serves no purpose to leave it locked in view if you can't interact with it in any way.

Since it's part of the VRC UI it overrides brightness gimmicks you can do in the world or on an avatar. So you get bright white arrows locked to the HMD on a darkened background; this could legitimately risk causing burn in for anyone who might pass out with the headset on and has their controllers die.

This has been an issue for a long time but I haven't seen it properly documented before.