What seems to be happening is physics will go to sleep.
When the character moves, there’s nothing bumping into it to wake it back up.
A swinging sign by comparison is woken up by things bumping into it.
A weapon on a character needs to be bumped by something.
If an object is on your back, and you walk forward, there’s nothing to bump it awake.
So I made a hollow box (can be hidden) which so far keeps it in place.
Although it has the odd moment while idle where the character moves slightly and it doesn’t. So not 
yet but I’d say this explains what’s going on.
A simpler box with the top and bottom faces removed will also work.