STemWin stops working after 20 ms

[MPlan.7]

I am using the STemWin BASIC_HellowWorld Application.

I modified the source code to look like this:

void MainTask(void) {
  char counterstring[128];
  int counter = 1;
  
  GUI_Clear();
  GUI_SetFont(&GUI_Font32_1);
  while(1)
  {
    HAL_Delay(1);
    sprintf(counterstring,"Time %d",counter++);
    GUI_DispStringAt(counterstring, 0, 0);  
  }
}

It prints out 13 times. Then emWin stops updating the display, even though the debugger shows the process is still running and the counter is incrementing, If I change the HAL delay to 2, it prints 9 times; if I change it to 3, it prints 6 times. If I change it to 0 it prints 139 times.

It looks like emWin just stops working after ~20 ms. Any clues, suggestions, or hints?

EDIT: I am using the STM32F769-Discovery kit with a 30-day evaluation version of IAR.