I wrote a small loop 100 to zero to go through the results of different DEFAULT font sizes.
The text passed is "Now is the time for all good men to come to the aid of their county 0123456789"
As you can see from the attached screensaves, font sizes 30 and 21 return something weird.
It changed the alpha text to all zeros. All the other font sizes are correct.
The top line is the font string with "W" length of one line and "H" height of line.