If a user chooses the pages he creates to be automatically added to his watchlist, once he creates pages through an extension (such as creating User Pages using the CheckUser's mass block feature), these pages are not added to his watchlist. It appears that watchlist addition doesn't work if pages are added through a hook, and not through edit.php directly.
See also: T36838: 127.0.0.1 in CheckUser log: Editing APIs should never use FauxRequest