One of the less well-known quirks of the 2014 Motorola-built Nexus 6 is that some third party applications show an upside down image when using the front facing camera. The reason for this is because the camera sensor is mounted in what's called a reverse landscape orientation and Google's old camera API requires application developers to explicitly set the preview rotation. Most devices use the forced landscape orientation and as such, the default setting works great - many applications therefore, do not call the display orientation method because developers did not call upon the API to check. It's these applications that do not work correctly with the Nexus 6's front-facing camera. However, because there are relatively few third party camera applications that use the front facing camera and the old camera API, this is not a widely known issue.
This is likely to change as the Google Nexus 5X also uses a reverse-landscape sensor orientation, which means these third party camera applications show an upside down image and preview. For those people lucky enough to be using the Google Nexus 5X and seeing a strange upside down issue with various photography applications, the reason is not an incompatibility with Android 6.0 Marshmallow, but something easier to be fixed. Android's camera framework tech lead took to Reddit yesterday (Saturday) to explain the reason why the camera sensor mount is essentially upside down: otherwise, the wires from the sensor chip would not have fitted in the limited space inside the 5X's chassis. Luckily, Google's revised and improved camera2 API, which was released at the same time as the LG Nexus 5 in 2013, works correctly with different hardware configurations.
A fix to the original camera API would cause those third party applications that currently work to show images upside down and of course it would be better to encourage developers to move to the newer camera2 API, so instead Google and LG are reaching out to developers to let them know of the problem and how to fix the issue. Google's tech lead has asked that customers also let developers know: the more people emailing or Tweeting developers about the issue, the more encouragement there will be to have these issues fixed. And as we have already said, migrating an application to the new camera2 API and making the correct calls, is also a good thing.