In acp_forums.php in function delete_forum_content() there is a do-loop to only delete a limited amount of data in one query, deleting the full data set in several iterations.

However, the outer do-loop only performs one iteration because the continue condition is always false.
Inner loop: while ($row = $db->sql_fetchrow($result))
Outer loop: while ($row)

MySQL is not affected because multi-table deletion is being used.