I did HAVE the same problem and it was resolved today by performing a reset. Settings, General, Reset, Reset all Settings.
Erasing and restoring from backup is not needed, and might restore any unwanted sensor recalibration settings. I believe what happens is the camera retains calibration settings that are extreme, such as when the iPhone is laid down on a table while the camera is active, and then the camera seems to adjust an exposure calibration setting that is retained afterward. All sensors are inherently different, especially when it comes to signal noise, and must be calibrated, and it seems iPhone adjusts these calibration presets over time as the sensor ages and may be adjusting at the worst possible moment. I will wait a few days to allow normal use, then update if I can reproduce this calibration problem later.