Nexus 6P - cannot turn off Developer Options
I suspect this strange behaviour is caused by some setting that requires developer options to be enabled, but itself is not deactivated for instance USB debugging., transitions/animations etc.. (this has been observed to enable developer options in some LG builds typically when the battery saving feature disables animations at 15% )

As a workaround try this:


Go to developer options, turn off debugging,
Then disable the  developer options with the switch on top of the screen
(same as your screenshot)


The trick is to first deactivate setting that uses developer options then toggle it off