The voltage is 2.6V only when you are shorting the divider output to the pin 4? Is it 3V when you are not shorting the output point to the GPIO? One more question, did you select "High impedance analog" in the drive mode option ?
Few things to check-
1. What is the minimum zener current that needs to be satisfied for regulation?
2. Is there anything else on Pin P3.1 on the board which can load the circuit?
3. What is the device VDD? Device can get back powered via IO if lower by 0.5V compared to pin voltage.