Why iphone 7+ can take picture only after moving from Video or SLO-MO or Time-lapse mode to photo mode?

The pictures I take directly from Photo mode, Portrait mode, Square mode, and PANO are not saved to the iPhone 7+. However, I switch to Video or SLO-MO or Time-lapse mode first and back to the Photo mode to take pictures, and the pictures can be saved. Does anyone know how to fix this problem? Thanks.