Trying to multiplex voltage reference gives incorrect output
As stated in comments, issue has to do with supply voltage to device and fixing that screws with the logic levels requiring additional components which makes the rationale for using the device itself suspect.

Alternative: Use two low RdsON MOSFETS to change the bias on the TL431 and or short the reference to ground. 







simulate this circuit &ndash; Schematic created using CircuitLab