Every time when I take a picture or video with flash, it turns out very, very bright on my iPhone 6s. Especially at night the glare is so bad you really can’t see anything.
Some cases do not leave enough of an opening around the flash LEDs. The edges of the case then reflect light from the flash onto the camera sensor, resulting in a lot of glare during flash photography and video recording.
The only solution is to not use that case while using your camera’s flash. It would be best to simply switch to a different case with a wider/better-designed rear camera opening.