Remember back in September when I posted about the fact that the Check for Update button on Google Pixel and Nexus devices actually worked? Like, it actually looked at the cloud where phone images live, did a stare-and-compare of what was there versus what was on your phone, and would immediately download it to your phone. It was cool for the few minutes it worked.
No, it is not your imagination. It is broken.
Googler Elliott Hughes has taken to his Google+ account to update everyone that the feature is broken due to an API incompatibility. It will get fixed but it won’t be until 2018.