I’ve run in the same issue by importing into PHPBB.
You can read through my post: https://bbpress.org/forums/topic/import-phpbb-wrong-authors/
If you check, you may find that some authors are displayed as anonymous because the corresponding attribute in wp_posts.post_author has no match with your wp_users.id
I’ve found out that these authors have still the old user id of the forum you are importing from (VBulletin).
I can’t guarantee the following solution will work for you, but I recommend you to create a temporary table to map your old user ids to the new wordpress ones by joinin them on the user_email attribute.
Then just update all the orphan wp_posts.post_author with the right wordpress id.
I can’t guarantee this will fix all your dirty posts.
I leave this reply for those who faced the same issue and also for the developers who may find where the problem is.
From one side we have a wrong author getting associated to the wordpress posts (forum, topic and replies).
One thing that can be done to fix this after having run the import is to match the date and time of the post, the ip address of the author and potentially the last 8 characters of the content (will not work if it’s a smiley or bb code).
This would fix the issue even if I can’t guarantee it will work for all the dirty posts.
The second thing, even more worrying, is the fact that certain authors are associated to the topics/replies with their old PHPBB user_id.
It’s easy to spot them because those dirty posts won’t join with the wp_user table and therefore they are reported as anonymous users in the website.
One way to fix that after the import is to update all those old PHPBB ids by replacing them with the corresponding wordpress users ids (just join on email address).
