According to a message posted in the Google Pixel support forum by a Google product manager, Google is currently looking into an issue with the 'double tap to wake' feature on the Pixel and the Pixel XL. The message comes on the back of numerous Pixel owners reporting that there are issues with the double tap and lift to wake features that were recently introduced to the Pixel and Pixel XL via the Android 7.1.1 update.
The response in the forum by the Google rep does not go into detail on the issue other than to recognize that some users are experiencing the issue and that "the team is investigating" and is "working towards a future fix". However, the issue being noted by users seem to revolve around the general inconsistency of the feature(s). Sometimes it will work and sometimes it won't or that there is sometimes a delay between the activation and response of the feature. At the moment, there are no details on when such a fix will come through, but if you have been experiencing the issue, then you can at least take some solace from knowing that Google is aware of the issue and is looking to remedy it at some point in the future.
Of course, this is not the first software issue that has been noted with the Pixel and Pixel XL, as users have already reported quite a range of issues including more than one with the camera, as well as an issue with the audio. Issues which again, have been recognized by Google and are either being fixed or are intended to be fixed via a future update. While it does seem as though the number of Pixel-related issues are starting to become quite high, it is worth keeping in mind that this is effectively a debut smartphone from a debut smartphone line and there is likely to be a number of teething issues which will have to be ironed out. If you would like to find out more about the issue in general, you can head though the source link below and over to the Pixel User community post on the topic. Likewise, if you are experiencing the issue, the latest Google message on this asks if you can use the "submit feedback" feature on your Pixel or Pixel XL (Settings > About phone > Send feedback) to keep Google informed about the issue you are experiencing. Or any issue with the Pixel or Pixel XL for that matter