Plumbing Defect

Tub spout leaks only when the shower diverter was on.

Yep, It’s a defect.

Can happen if the diverter mixy thingy fixture is installed upside down.

Happens more often when they improperly use PEX or CPVC between the tub spout and the valve. Need to use Copper or Galvanized, as per the valve spec.

Good information Dom.