When you say "it stopped working" do you mean the nomad charger or the usb data port? I haven't had any power delivery issues with my nomad using these cables; but I have ocassionally lost my USB in the tesla interface which required me to stop the car then start it again before it would read the USB drive again. Only has happened once and I'm not convinced it was the splitter cable as the connection actually seems fine and I didn't replug it or anything for it to recover. I think it was probably a tesla bug.
You said you left the car... I think the USB ports stop working when you leave the car anyways. Could it be this behavior?
I plug in the Nomad USB cables into the two ports: works. Charges fine. I then unplug the Nomad USB cables, plug in the two splitters (I bought 2, one for each port), plug the Nomad USB cables into the red side of the splitters. Sometimes it works, sometimes it doesn't. I have to do a few unplugs and plugs and then it'll work. Once it works, I test by removing my phone off the charging pad back and forth, and it charges fine, doesn't skip a beat. I then get out of the car, close the car door, and the screen shuts off. I reopen the car door, waking up the car computer, then try to charge my phone again, and it doesn't work.
I don't think the splitter cable is faulty. I'm just not why it works then stops. I'm thinking the Software version that I'm on might play a role? We'll see. I won't stop trying.