My printer is Cartesian. This is a test rig to work the kinks out of my configuration before I deploy these sensors to my small (but growing) print farm. All of the farm printers are also Cartesian.
Changing the leveling method in Marlin was the ticket. I enabled linear leveling instead of bilinear and I got a perfect first layer! I went back and enabled 9 and 16 point bilinear leveling and tried it once more after having some success with linear bed leveling. Unfortunately, as soon as I ask my printer to calculate a complex mesh, things get wonky. So, I think we've solved the mystery. It was never the sensor's fault. The board wasn't performing the calculations correctly in the more advanced bed leveling options. Taking away most of the math and using only four points has enabled it to pull through and create a proper plane.
Thanks very much for your assistance. I will continue to buy and recommend Duet electronics. I can't wait to get my Duet Wi-Fi for my larger printer!