I've noticed this as well and @SimGuruModSquad has just seen my post about it. The metal parts are the stereo and aromatherapy upgrades and so SHOULDN'T be visible when the object is first placed, but only once the upgrade has been added.
In my case of this bug, the upgrades don't display when the hand tool holds it before placing and they appear when I let go.
If I get a Handy Sim to add one of the upgrades then the other vanishes but will reappear if the Handy Sim then adds the relevant upgrade.
BOTH tubs do it in my game.
