I tested on a custom ROM, Mokee 71.1, but with the same bug: flipped image (tested on Open Camera and Manual Camera). I couldn't find a fix, I guess nobody seems to be interested to fix this particular bug (or maybe it's not possible to fix it).
I read xiaomi mi5 users after enabling camera2 api have flipped images on front camera (xiaomi 4c has issues with back camera) and fixed that editing a camera xml conf file. That file was on /system/etc/camera/ but on mi4c that path is missing.
There are many open thread on miui forum about requesting camera2 api support for many devices that could handle it. Xiaomi don't abandon easily development for its devices but do not listen to request.