I think you might have to contact all the instances yourself, depending on what the relationship between the instances is. Neither instance is really contracting with the other for data processing; it's more like one instance publishes something and the other instances download and republish it, and everyone agrees that that is what they are supposed to do. So if you and your affiliates have to delete someone's data from a GDPR demand, it can't really apply to just other people who copied it?
I am, of course, three European lawyers in a trench coat, and this is impeccable legal advice that physically cannot be wrong.