The resource of this user is acct:[email protected]. Check the user profile settings in WordPress to check the WebFinger resources for the other users.
I am having a similar struggle with this webfinger appearing/ being missing.
I just worked with the dev’s at WPMU to test it out. We tested with the ActivityPub plugin, without it. With security plugins, without them. With XML on/off. Changing pretty print permalinks from one to another. Adding .well-known directory in file system.