I feel a bit stupid, as I am sure I have seen it somewhere mentioned in a response to one of my earlier posts, but I can't find it anymore.
If I deploy a Unity app, it works fine, but during the splash screen it shows a black background, essentially dimming nearly the whole environment. I know you can turn that off, but I don't remember how.
You will need to disable Segmented dimming by changing the Blend Mode of your application. This happens because Unity does not submit the correct alpha values in the splash screen. My recommendation would be to create an app that targets the Additive Blend mode and then activate the Alpha Blend once the app is started.