In phpbb, the attributes of a topic can be changed by editing its first post.
=> The topic subject is herited from the first message. If someone edits the first message and changes the subject, the new string also becomes the topic subject.
=> Same applies for icon. If there is an icon change for the first message, the same change also applies to the topic icon, visible in forum pages (viewforum.php) or in search results (search.php).
However, when a new topic is created through the moderator control panel (MCP), by splitting a former one, the new icon applies to the newly created topic (as expected) but not for its first post.
Therefore, the new icon may not appear on the topic page (viewtopic.php). Moreover, if someone edits the first message, he or she can remove or replace the topic icon without even realizing (because it isn't selected by default on the post editor).