I am using a factory unlocked/unbranded Nexus 6 for a few days now and have not seen this issue...yet.
Nexus 6 is an absolute pig of a phone, weirdest nexus ever made. The giant phone that should not have been made. Full of bugs and dodgy burn in screen. Should all be dumped.
Anonymous, 11 May 2015Mine N5 black 32 go has developed same problem not registered sim on network after update to 5... morebuddy i've seen multiple issues of the 5.0.1 update buggering up different phones - most commonly on the samsung S5 and Note 4 - causing loss of connection, no access to text messages, power failures i.e not booting properly, and issues with memory and such like.
Try a full restore after backing up your device - it might help you