Steps to reproduce:
1. Open any project
2. In player settings set default orientation to LandscapeLeft
3. Build the project
4. In player settings set default orientation to LandscapeRight
5. Build the project to the same location
6. See that SplashScreenImage.jpg file is facing LandscapeLeft direction

By design because:
Unity has no way of knowing whether the SplashScreenImage.jpg was replaced with custom one, thus it does not modify it if it already exists. Four images, one for each orientation, are put to output directory. If there is a need to change the default splash screen image orientation just rename a file to which you want.

If there is a need for Unity to recreate the images, delete all the images created by Unity ("SplashScreenImage.jpg", "SplashScreenImagePortrait.jpg", "SplashScreenImageLandscapeLeft.jpg", "SplashScreenImagePortraipUpsideDown.jpg" and "SplashScreenImageLandscapeRight.jpg") and Unity will recreate them according to your player preferences upon next export.