Check out this video of Mavis Co-Founder, Dr Phil, as he dives into the Monitor Tools (including Flip) inside Mavis Camera:
The Flip tool can be used to flip the orientation of the viewfinder preview.
Further settings can be found by tap and holding down the flip icon on the monitor toolbar or via the setting menu. You can flip both Horizontally, Vertically, or both (rotate 180).
Flipping is anchored to the device orientation, as such the icon will update based on whether the device is held landscape or portrait.
Other orientation information can be found within the Orientation Settings article.