I’ve had the same issue (icon changed back to the default one, even when I had my paths set correctly), changing the line
<preference name="prerendered-icon" value="true" />
in config.xml from true to false solved the problem for me.
I have faced the same problem, and despite searching for a definitive I had no luck trying to add the icon tags in the config.xml, neither by storing my icons on the www/icon folder so cordova build could copy them as probably expected. The documentation is not clear at all, scattered around different posts and solutions.
So what works for me is the following, although I think is not the most elegant solution but it works.
Open your XCode project and store your icons on the folder Resources/icons or alternatively select your target project, go to Summary and scroll down until seeing the icons, click on any of them and right click on it and select open in finder so you have the correct location to override the default ones.
Be sure to use the same names that are used in the target folder to avoid confusion and CLEAN your project and BUILD it again.
It is likely that if you Run the project in the simulator or device the default icon appear, in that case just uninstall your app from the device and build it again.
Hope it helps, however I would expect to have instead a method to modify the development www and let cordova build copy the icons and splashcreen.