It’s going to be something in that PostProcessVolume. Try deleting it completely. You can bring it back with CTRL+Z right after. If it changes to what you’re looking to get, you did it! If not, I’m thinking it could be auto-exposure.
To see if that’s it, select your PostProcessVolume and in the details section search “Brightness”. There will be a minimum brightness and maximum brightness variables, enable them and set them both to 1.0.
Then make sure the post process volume also encompasses your camera, or is set to Infinite (unbound) and give that a shot! 
Regardless, the problem has to lay with something the Post Process Volume is doing, OR something it can fix.