When you delete a topic from somebody else , it will result in that the author is not displayed on the logs.
For example;

You delete a topic from person b, the topic will be removed successful.
After that you going to visit the mcp , and you see the next log action;

Deleted topic “Test” written by
»

The author is missing.

Looks to me a regression of http://code.phpbb.com/repositories/revision/phpbb/10170