Performing some log actions (move, protect, upload) also inserts a fake revision into the page edit history, with a summary identical to the log comment but with a prefix like "The page was moved:". In some cases, this prefix causes the summary to exceed the length limit and be truncated.
Given that the length limit is currently 500 characters (CommentStore::COMMENT_CHARACTER_LIMIT), and the database field gives us space for at least 16383 (CommentStore::MAX_COMMENT_LENGTH / 4), it seems we should be able to avoid this.