I found the answer as I was typing up this question but I’ve been wrestling with it for a while now, so posting in case it helps anyone else.
What solved it was changing the TextSpan line to:
TextSpan span = new TextSpan(style: new TextStyle(color: Colors.grey[600]), text: 'Yrfc');
Apparently it was either drawing the text invisibly or as white (background) color since I hadn’t made my color choice explicit.