Switching to ‘Fullscreen’ window mode in a package game in 4.14 causes the gamma to change significantly. This has made it impossible for us to ship in 4.14.
To repro, simply create a project in 4.14 and package it. It will start in windowed mode. If you use a console command or scalabiility setting to change the window mode - the gamma will increase in brightness.
So I just ran a test on my end in a new blank project, packaged for windows (64-bit), ran the game, took a screenshot, set it to fullscreen mode, took another screenshot, and compared those results side by side. Below are the results I received after testing.
These two images are nearly identical, and I just wanted to share my results. Perhaps I am missing something, but I would check to make sure you do not have any sort of gamma correctness or post process volume settings being applied, more specifically the ‘Eye Adaptation’ or ‘Auto-Exposure’ setting.