I'll offer this up in case it helps. On Win10, using a Standard (Non-Admin) user (with UAC enabled as well), I have to apply icon packages twice to get them applied:
1st time - Open up Icon Packager using the right-click "Run as Admin" option. Apply the icon package and close Icon Packager. (This of course also changes the icons on the Admin user.)
2nd time - Open up Icon Packager normally. Apply the icon package and close Icon Packager.
Don't know if that's what some people are running into, haven't seen anything about it, but that's how it works for me.