Make sure that the name of the file in your res/icon/ios/... directory matches exactly (case matters). Also double check that the resolution of the image is indeed 87x87. I've run into cases in the past where the image wasn't the size as specified in config.xml and as such the icon wasn't used.
I have same problem when my config file uses :
<preference name="phonegap-version" value="cli-6.3.0" />
But problem goes away when using:
<preference name="phonegap-version" value="cli-5.4.1" />
So I will not upgrade yet to latest version till this is explained or fixed, anyone else get a proper fix?