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It might be hardware issue, kindly dial *#*#64663#*#* and check front camera is working or not. If working then kindly Re-update your device to latest global stable ROM. If not working then visit your nearest Mi authorized service.
I'm also facing this same issue front camera not working it's button appears for 1 millisecond and then vanish, i was using miui10 beta latest and thought it might be a beta bug as firstly in miui10, at first front camera icon was appearing was not working just crashing then later back camera started doing this too, so I decided to shift to stable rom which still is miui9 and erased my whole 50gb & above to correct this and only to find the problem exists in miui9 also, even worst here it is not even showing button for front camera.