It could be you have sensor failure like I'm experiencing. My accelerometer, gyroscope and compass sensors were broke by Google with the update to 10. This was verified by a sensor test app I installed called "Sensor Test". Accelerometer or gyroscope sensor failure affects the issue you present, the double flip gesture to switch to selfie mode not working when the camera app is open, also other gestures not working like auto rotate, flip to shhh that enables do not disturb and other issues. Can't even take a panorama picture or photo sphere because of the sensors not working. Very frustrating on a 1 yr old phone. Bunch of other people are experiencing this. I'm following 2 different Google support threads on sensor failure after update to 10. Google has no answer for this. I can get the sensors to work for about 5min by restarting my phone, then they stop working. I suspect many more people don't realize their sensors are not working because normal everyday activities like phone use, email, text, wif-, bluetooth do not use sensors or seem to be affected by sensor failure. It's only when you realize you wanted to have your photo or video auto-rotate when you turn your phone sideways and it doesn't, or your panorama mode on your camera doesn't work, or you can't slience your phone by turning it screen down onto a flat surface, or, as in your situation, you can't flip your wrist twice to engage the selfie camera.
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