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AD7687 Operates Single-Ended, not differential.

I have a board with three AD7687's that are acting like single-ended ADC, not differential.

The parts are marked C03 #535.
The devices operating in Chain Mode With Busy Indicator, connected as shown in the datasheet.
Sample rate is 5K samples/second.
SCK period is 110ns (9MHz) 50% duty cycle.

VDD and VREF are both 5.0 Volts. VIO is 3.3V.

I have verified all signals with an oscilloscope and all clocking looks valid, and the data being read by the software matches what I see on SDO with the scope.

All three devices are acting similarly. The following data was collected from one device, and is repeatable.

With a differental input:
IN+ at 1.8V, IN- at 3.25V (a differential of -1.45 V) I get an output around 15XX Hex.
IN+ at 3.7V, IN- of 1.7V (Differential of +2.0V), Output is around 55XX Hex.
Varying between these two conditions, I get ONE intermediate step around 2BXX hex.

If instead, I operate single ended:

IN+ 1.8V, IN- GND, Output is 5BXX.
IN+ 3.6V, IN- Ground, Output is B7XX.
These outputs are about what I would expect from a single-ended ADC.
Also, when I vary between these two conditions, I get nice uniform steps as would be expected.

Going one step further, Grounding IN+ and applying any positive value to IN-, the output is 0000.

Anybody have any clues? Or are my parts not really AD7687's?

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