Android does not allow directory creation with SDK update

Strange issue, but on one device Android no longer lets the app create a directory (or any directories). This seems to have happened since I moved to SDK 27 (I brought in a 3rd party library that requires the upgrade). Even stranger, it works on other devices. The device that does not work is a Samsung Galaxy Tab S3, running Android 8 (sdk 26).

Here is the code that fails:

public static String mediaStorageDirectory() {
    return Environment.getExternalStorageDirectory().toString() + File.separator + "myapp";
}

private DatabaseHelper(Context context) {
    super(context, DB_NAME, null, 1); 
    File dbPath = new File(Utilities.mediaStorageDirectory() + "/databases/");
    if (!dbPath.exists()) {
        if (dbPath.mkdirs()) {
            this.mContext = context;
            createDataBase();
        }
    }
}

The code creates a directory, then copies the app DB in into that directory. This code has been working since the dawn of time...has Android changed some security requirements for file creation?

If you are wondering, I have set privileges as follows:

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.CAMERA" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />

And the user is prompted for those privileges successfully before the creation of the directory structure.