Why won't my Nexus 6p display timeout as set?

[AC Question]

Whenever I activate the screen to use my phone the display stays active despite a 15sec timeout being selected.
I have found the screen still active 20mins later after having put it down and have to constantly turn the screen off via the power key.

The phone is in a wallet case and there does not appear to be any interference from the button protectors.

Thanks