Issue is not based on your particular phones or hardware defects, I advise you read http://forum.xda-developers.com/nexus-6/devs-only/speaker-amplifiers-nxp-tfa9890-research-t3236518 . Our n6 amplifier chip is weird and also qualcomm drivers is fo...
I used this mod on lollipop and it is awesome, but It stopped working in marshmallow . Is there any chance that you will revise it? I think problem is assist app function come with marshmallow.
FYI regarding double tap to wake, you need to double tap to wake phone then double tap again to release ambient display, then swipe up to unlock to phone.
It is weird but also seems logical when you just want to check time but not wake the phone ...
AT&T has a new conenction they called 5G evo but actually it is 4G not 5G and only certain phones has capability to recevie this signal(iphone X series and qualcomm X24 modem which is Pixel 4 not 3 unfortunately)
Think of it as a Lte advanced but...
Similiar error happened to me when I was using old version of adb tools. Try to find last one or better yet download android sdk platform tools, which will ensure you will have lastest version of fastboot and adb.