@grishka The way Mastodon hacked around this, and other implementations have copied, was by taking another element of the actor, `preferredUsername`, and the domain part of the actor endpoint, and making a Webfinger id out of those two. So, in the above example, it'd make a Webfinger ID out of potus@whitehouse.example.
@grishka It then does the Webfinger lookup again with that new Webfinger ID, and checks that it points to the right Actor endpoint. It then stores this webfinger ID as the right one to use for this actor from now on.