Back in October, we reported on a change in the Android 8.1 release that made the proximity sensor behave differently. Unlike before, it wouldn't turn Ambient Display off when you covered the sensor, something we saw even in 8.0 for the newer Pixel 2s. That may or may not have been intentional, but it seems there were also other issues relating to the proximity sensor in 8.1, more about which you can read in this mega thread on the Pixel User Community forum.

Many of the problems specifically affect the Pixel 2 XL, and range from the sensor being confused by things like screen protectors, causing 'always on' Ambient Display to turn off unexpectedly, to not being able to use the device at all during calls.

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