When exporting facing pages with mirrored background images and gradient settings, all left pages seem to be darker and slightly warmer. The files are placed and linked to the same image from my hard drive and even swapping each image left to right causes the same bug. I even made a new parent page from scratch with the same issue. Pages look exactly the same in Indesign preview. The issue reverses when exporting a JPEG, where the left page is lighter and the right is darker.

Indesign CC