Hi Ron,
Let's back up a bit. There are only a few things that can actually affect the measurement. One is a varying input and another is the reference. The are others of course, but those are the two main considerations. I apologize that I read your post so quickly. You said you held the MCU in reset. How did you determine the output? Or, are you externally measuring across the resistors?
As I don't have a schematic, I'm only guessing at how you have things connected and at what points you are measuring on your resistor string. You originally stated that the board warms up. Is the board in a room temperature environment or is board in a temperature chamber?
I apologize for the datasheet. This was undoubtedly a cut and paste error when creating the datasheet for the ADS1242/43. The ADS1240/41 has the buffer enable pin.
Now back to my original comments. You asked if there were in any way the devices could cause an issue. Remember that the values you quote are impedances and not resistances. The inputs to the devices have switched capacitor inputs that switch in and out of circuit at the modulator rate. There will be some movement of charge. When the charge is available relative to the desired input voltage will determine what is actually measured. Depending where the input voltage is relative to the reference, there actually can be charge that kicks back from capacitor as a discharge as well as charge. You are most likely correct that the severity of the problem is not the impedance.
With the ADS1243 there is one other possibilty for an error and that is if the burn out current sources are accidently turned on. There are most likely some other potential issues as well, but given the limited information it is difficult to tell. If it is a single issue or combinational.
Best regards,
Bob B