Voltage leak on ADG1406BCPZ


I am using an ADG1406 (12Vdc) to multiplex voltages to the ADC of a Cypress Psoc5 microcontroller (5Vdc permanent). The 5V is also connected to the ADG1406 on S8.

The problem appears when the 12V is down (so when the ADG is OFF), the 5V (which is permanent) connected to S8 is found on the VDD pin of the multiplexer. The 5V is therefore connected to the 12V line of the card. How to explain this?

Thank you,