I bet you're right.My theory is that by taking my phone to the store and letting them not just install the SIM but register the phone with its IMEI, they basically turned my unlocked device into a carrier phone. For past unlocked phones (not Nexi) I would just put the SIM in on my own and call it a day with no issues.
Only reason I went to the store this time was because I didn't have a nano SIM :-/
AC App
Nexus 6
Both of my friends with this phone on AT&T haven't received the update. One bought it unlocked and walked in for a nano, the other is carrier subsidized.
I'm so glad I flashed (on October 9th!) or I'd be in the same boat. I'd have probably gone crazy waiting another two months! Seriously folks, it's worth any pain to get Marshmallow. The battery improvement alone is like having a new phone...