When set to Sanitize/Winterize, city water flows to the input side of the pump (which is also connected to the fresh tank). Normally when adding bleach or antifreeze, it's sucked in by the pump. But city water is pressurized. So some of it will flow backwards from the input side of the pump, to the tank, causing an overflow.
Seems awfully coincidental that they think the faceplate is wrong and that sanitize/winterize is really city. The faceplate is stamped and can't be wrong. But the cartridge could be installed in the wrong orientation. If it were, when set to city, you'd really be in sanitize/winterize and would be filling the fresh tank. That being the case, if you switched it to sanitize/winterize, it should resume normal operation.
The cartridge can be easily removed and re-inserted correctly if it's actually in wrong. Maybe you need to go to CW and show them the service document for the valve.
Here's a link.