Could you describe in more detail the procedure for this to happen? "click a picture" means "Touch shutter" enabled? In selfie -> Portrait Mode? Was Beautify enabled or disabled? (Lower right corner icon). Was artificial bokeh enabled?
By increasing brightness I assume you used the slider after touch for focus. Do you get this every time?
Can you try to clear your Camera App settings (long press camera app icon, go into its settings, clear data & Cache) and then try again?
I barely get this issue and I just wanted to know whether people are there whose facing this issue. Lastly when I faced this was like last week. And just few minutes back I had it again. Cuz I don't use portrait much.