Voltage error when connecting a zener diode
The values of resistor R17 is too high, this means only small currents can flow. The zener diode isn't ideal, it also leaks current. Perhaps the current through R17 and the zener's leakage current are in the same order of magnitude preventing "good zener" behavior.

Let's calculate:

When Vbat = 4.2 V and the zener would work there would be 3.3 V at the output. So across R17 we get 4.3 - 3.3 V = 0.9 V. That means a current of 0.9 V / 127 k = 7.1 uA.

Hmm, only 7.1 uA, that's quite close to the leakage current of the zener diode. See \$I_R\$ in the table in your question, it is 5 uA so quite close to the 7.1 uA that would flow through R17 if all worked well.

Also note the value of \$I_{ZT}\$ in the table, it is the current which the zener diode needs to have a proper zener behavior. Using a higher current is also OK. To get the 5 mA  from the table, the value of R17 needs to be substantially lower. In the order of 180 ohms !