Re: two new shower cartridges leaking..strange
A few things come to mind. It is possible that the replacement cartridge was defective, not likely, but possible. The replacement cartridge could have been damaged or improperly installed. Since this is a new house, it is possible that the valve body was damaged during the installation when the house was constructed. If this is the case, then the only repair is total valve replacement. For the time being, I would recommend changing the cartridge one more time and see what happens.
FYI, changing a cartridge is easy, fast, and the parts are generally cheap, something you can do for yourself with basic tools.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!