I figured out why this is not working. The iptc profile is actually stored inside the 8bim profile. And the changes in the IPTC profiles are not stored in the 8bim profile. I will have to add this feature/fix to ImageMagick for you.
A workaround for now would be to also remove the 8BIM profile. This will create a new 8BIM profile that contains your new iptc profile.
A workaround for now would be to also remove the 8BIM profile. This will create a new 8BIM profile that contains your new iptc profile.