With the iPhone 7 Plus Simulator I see the correct splash (portrait or landscape) being loaded depending on whether the device was in portrait or landscape when the app is started. For apps that only allow portrait the Landscape splash is never used in my tests.
This is how the launch-image plist configuration is set up:
<key>UILaunchImageFile</key>
<string>Def.png</string>
<key>UILaunchImages</key>
<array>
<dict>
<key>UILaunchImageMinimumOSVersion</key>
<string>7.0</string>
<key>UILaunchImageName</key>
<string>Def</string>
<key>UILaunchImageOrientation</key>
<string>Portrait</string>
<key>UILaunchImageSize</key>
<string>{320, 480}</string>
</dict>
<dict>
<key>UILaunchImageMinimumOSVersion</key>
<string>7.0</string>
<key>UILaunchImageName</key>
<string>Def-568h</string>
<key>UILaunchImageOrientation</key>
<string>Portrait</string>
<key>UILaunchImageSize</key>
<string>{320, 568}</string>
</dict>
<dict>
<key>UILaunchImageMinimumOSVersion</key>
<string>8.0</string>
<key>UILaunchImageName</key>
<string>Def-667h</string>
<key>UILaunchImageOrientation</key>
<string>Portrait</string>
<key>UILaunchImageSize</key>
<string>{375, 667}</string>
</dict>
<dict>
<key>UILaunchImageMinimumOSVersion</key>
<string>8.0</string>
<key>UILaunchImageName</key>
<string>Def-Portrait-736h</string>
<key>UILaunchImageOrientation</key>
<string>Portrait</string>
<key>UILaunchImageSize</key>
<string>{414, 736}</string>
</dict>
<dict>
<key>UILaunchImageMinimumOSVersion</key>
<string>7.0</string>
<key>UILaunchImageName</key>
<string>Def-Portrait</string>
<key>UILaunchImageOrientation</key>
<string>Portrait</string>
<key>UILaunchImageSize</key>
<string>{768, 1024}</string>
</dict>
<dict>
<key>UILaunchImageMinimumOSVersion</key>
<string>8.0</string>
<key>UILaunchImageName</key>
<string>Def-Landscape-736h</string>
<key>UILaunchImageOrientation</key>
<string>Landscape</string>
<key>UILaunchImageSize</key>
<string>{414, 736}</string>
</dict>
<dict>
<key>UILaunchImageMinimumOSVersion</key>
<string>7.0</string>
<key>UILaunchImageName</key>
<string>Def-Landscape</string>
<key>UILaunchImageOrientation</key>
<string>Landscape</string>
<key>UILaunchImageSize</key>
<string>{768, 1024}</string>
</dict>
</array>
Does this match your plist settings where the wrong splash is shown?
Best,
Günther