After sideloading Android 8.0 Oreo on my Nexus 6P, I found that Android Pay no longer considered my device secure. There’s apparently a bug in the OTA version Google posted publicly on their website, OPR6.170623.017. Luckily, the OTA that went out to most people, OPR6.170623.019, does not have this bug. You can wait for an OTA notice from Google, which might take a couple of weeks, or you can sideload OPR6.170623.019.
If you are running a version of Android other than OPR6.170623.017 DO NOT install the above-linked update