Why does the viewfinder view on the Lightroom camera look different than the resulting photo?

I have recently started taking pictures with the Lightroom app camera, in the "Pro" mode.

When I am about to take a photo, the subject looks properly exposed on-screen (not too dark or light), and after I take the photo, the little proof of the photo that appears on the bottom left of the screen looks fine. However, when I go to the album to look at the photo, it looks either way lighter or way darker than when I took it/the proof. 

This happens both when I'm using auto and when I manually change things (ISO, shutter speed, etc.). Auto has been resulting in the picture being way to bright and manual has been resulting in the photo being too dark.

Does anyone know the reason for this, and/or ways to make it so I know what the photo will actually look like before I take it? Thank you!