Every bathroom faucet in my house is leaking in roughly the same fashion. They were installed roughly 2.5 years ago (new construction). They are connected via Pex lines. The leak is inside the threaded, brass pipe that comes down from under the handle.
The odd thing to me is it's every single bathroom faucet (6) on both the hot and cold side. I can't see any way to tell if it's the pex connection and I don't see anything wrong with the cartridge. It is a slow drip-leak out of the inside of the brass coming down.
What I also don't see is any sort of O-ring or washer.
These cartridges are like 25/each so having to suddenly replace all 12 on relatively new faucets is kinda suspicious.
I believe this is the parts diagram for it: https://media.deltafaucet.com/PartsDiagram/DPD-L-2538-DST.pdf