"common sense" is not the issue here. What would have happened if the op got up to stow their bag and turbulence hit causing the op to injure themselves or someone else?

I think the correct course of action in this case would have been for the FA to take the bag to a locker/cupboard where it could be stored safely and without the FA having to lift it (FA's do this all the time). And if there was no space in such a place ? I don't know. Which rule takes precedence?