The thing about updates is that while it does fix some of the old issues, new issues will come up as well and for the Nexus 6 users, the update affected their Android Pay feature.
It was reported that the Android Pay feature stop working after the Android 7.1.1 update. Some reported that the app would not work while other said that their details went missing after the update. Some people tried fixing it by uninstalling and installing the app but that did not work.
Google did not acknowledge the issue publicly but it looks like they were aware of it after all as new people have commented that the issue was fixed after the 7.1.1 N6F26U update.
So if you are facing some issue with the Android Pay after the last update, you might want to check if there is a new update and see if that fixes the problem.