We’ve investigated this extensively, and since it’s a file/folder permission limitation, the only way to access the non-iTunes File Sharing folders is via Jailbreak (hacking the iOS file system). iExplorer gives you file system access, but it’s Apple who determines and sets the file and folder permissions, and they have blocked access to all app folders for App Store apps besides the Documents folder of iTunes File Sharing enabled apps. All other iOS transfer utilities will have the exact same limitation since it would require hacking iOS (Jailbreak) to break down the walls that Apple has put up in iOS 8.3. Most rumors point to a Jailbreak to be released when iOS 8.4 comes out which should be any day now. In the meantime, and if you’d prefer to not Jailbreak, the article mentioned above explains how to still transfer app files to your computer. Here it is again for your convenience:
App folders in iOS 8.3 are empty. Why is that?