Ok, now I understand what was happening. -[NSMutableAttributedString fixFontAttributeInRange:] was getting called because my article list font, Helvetica, didn't contain the characters.
It was more of a problem for me, because I had a month's worth of articles in the list. I was getting long delays and sometimes beachballs when selecting articles or scrolling the list. I switched the article list font to Hei, and all the delays disappeared.