It is because miui gallery is not adopted for external camera folder location (because their phones simply don't have external card slots) so if you switched your photos storage location to memory card, nothing will be displayed in camera folder. In fact this is correct behavior because your camera folder on internal storage is empty. Miui simply does not know about this change and unfortunately there is no option to change default camera storage location in gallery setting. Maybe this could be a work for guys from miui android to implement this switch, shouldn't be so difficult to implement... Seems Xiaomi does not care as that switch is not needed for their phones :-/