Try to upgrade your phpBB forum before the import. When you chose to import the users, it should work fine as I checked. The only problem appear if you want just to import the posts without the users or if you delete the users, it show the author as “Anonymous” instead to show the username or nickname.
In version 2.6 alpha seems that the problem is solved, but just if you chose to import the users. When the users remain as guests, all shown as Anonymous and not linked to any topic. There is any soultion for this specific problem?