This normally happens when your launch image is not correct. iOS determines the screen size on the basis of size of launch image. I faced the similar issue for iPhone 6 , I fixed that issue by adding launch image as png in the application (not in images.Assets) and adding the UILaunchImages dictionary in info.plist.
But here too for iPhone 6 use landscape image. here is the source code of info.plist. But we sure that all the launch images are of correct size.
 <array>
        <dict>
            <key>UILaunchImageMinimumOSVersion</key>
            <string>8.1</string>
            <key>UILaunchImageName</key>A
            <string>Default-320x480</string>
            <key>UILaunchImageOrientation</key>
            <string>LandscapeLeft</string>
            <key>UILaunchImageSize</key>
            <string>{320, 480}</string>
        </dict>
        <dict>
            <key>UILaunchImageMinimumOSVersion</key>
            <string>8.1</string>
            <key>UILaunchImageName</key>
            <string>Default-320x480</string>
            <key>UILaunchImageOrientation</key>
            <string>LandscapeRight</string>
            <key>UILaunchImageSize</key>
            <string>{320, 480}</string>
        </dict>
        <dict>
            <key>UILaunchImageMinimumOSVersion</key>
            <string>8.1</string>
            <key>UILaunchImageName</key>
            <string>Default-Landscape-568h</string>
            <key>UILaunchImageOrientation</key>
            <string>LandscapeLeft</string>
            <key>UILaunchImageSize</key>
            <string>{320, 568}</string>
        </dict>
        <dict>
            <key>UILaunchImageMinimumOSVersion</key>
            <string>7.0</string>
            <key>UILaunchImageName</key>
            <string>Default-568h</string>
            <key>UILaunchImageOrientation</key>
            <string>Portrait</string>
            <key>UILaunchImageSize</key>
            <string>{320, 568}</string>
        </dict>
        <dict>
            <key>UILaunchImageMinimumOSVersion</key>
            <string>8.1</string>
            <key>UILaunchImageName</key>
            <string>Default-375x667</string>
            <key>UILaunchImageOrientation</key>
            <string>LandscapeLeft</string>
            <key>UILaunchImageSize</key>
            <string>{375, 667}</string>
        </dict>
        <dict>
            <key>UILaunchImageMinimumOSVersion</key>
            <string>8.1</string>
            <key>UILaunchImageName</key>
            <string>Default-375x667</string>
            <key>UILaunchImageOrientation</key>
            <string>LandscapeRight</string>
            <key>UILaunchImageSize</key>
            <string>{375, 667}</string>
        </dict>
        <dict>
            <key>UILaunchImageMinimumOSVersion</key>
            <string>8.1</string>
            <key>UILaunchImageName</key>
            <string>Default-2208x1242</string>
            <key>UILaunchImageOrientation</key>
            <string>LandscapeLeft</string>
            <key>UILaunchImageSize</key>
            <string>{414, 736}</string>
        </dict>
        <dict>
            <key>UILaunchImageMinimumOSVersion</key>
            <string>8.1</string>
            <key>UILaunchImageName</key>
            <string>Default-2208x1242</string>
            <key>UILaunchImageOrientation</key>
            <string>LandscapeRight</string>
            <key>UILaunchImageSize</key>
            <string>{414, 736}</string>
        </dict>
        <dict>
            <key>UILaunchImageMinimumOSVersion</key>
            <string>8.1</string>
            <key>UILaunchImageName</key>
            <string>Default-2048x1536</string>
            <key>UILaunchImageOrientation</key>
            <string>LandscapeLeft</string>
            <key>UILaunchImageSize</key>
            <string>{768, 1024}</string>
        </dict>
        <dict>
            <key>UILaunchImageMinimumOSVersion</key>
            <string>8.1</string>
            <key>UILaunchImageName</key>
            <string>Default-2048x1536</string>
            <key>UILaunchImageOrientation</key>
            <string>LandscapeRight</string>
            <key>UILaunchImageSize</key>
            <string>{768, 1024}</string>
        </dict>
    </array>