OK, I chased down my leak today. After removing the shower valves and locating the back flow preventer on the black flush line, I found that was not the problem.
But in the course of freeing up the back flow valve, I had to cut a zip tie that was securing it to the wall. This allowed the Pex tubing to droop a little. When I tried the black flush again, the water started pouring out again. This time I could see from the bottom that the Pex tube itself was split about 2" up into the access hole. The split was spraying water upwards, and then it would run out of the access hole.
I wrapped the tube with a rubber tape to hold it until I get back to the dealer to have it replaced. It's amazing, I have NEVER seen a piece of Pex split before. That stuff is like iron. Go figure.