Shower Tub Valve leaking from behind cartridge

I have a Delta shower/tub valve which controls water flow to our tub (with a diverter that can be switched to shower). There is a slow leak from the tub faucet which I am trying to repair. This valve was installed less than 6 months ago, so the leak is a bit surprising.

After turning off our water lines, I disassembled the valve and removed the cartridge. The cartridge and o-rings seemed ok and not worn (to be expected).

However, the leak was still occurring from behind where the cartridge was. On the right side of this image, you can see some water leaking from the cold water supply line. When this image was taken, the water supply was turned off and the bathroom sink did not have any water pressure or leaking.

• Is this the same leak that is causing my faucet to drip? (I am assuming so, with the water flowing through the cartridge somehow?) Or is the cartridge still possibly to blame?

• What could be causing a leak from further back than the cartridge?

• What can I do to fix this?