In the default template, the class for the potion item in Active Subscreen (New, B+A) is set to Potion, rather than Letter or Potion. So, I had to choose Subscreen (New, B+A), then right-click the potion item, and reset its item class to Letter or Potion. On the other hand, the class for the potion item in Subscreen (New, B) is set to Letter or Potion, which means it's set up correctly.