Okay, I guess those are technically ASCII. However, anything under 0x20 is an unprintable control character. That's what Notepad++ is telling you with the weird black three-letter blocks. The first character in the file is 0x02, otherwise known as the Start of Text control character, or STX. There is no glyph for that character, you can't print it out. The next one a little later is 0x17, End of Transmission Block or ETX. That's some seriously old-school teletype data you have there.
Notepad++ opts to insert a placeholder for unprintable characters, but the viewers in DevTest just leave it blank. However, in the binary viewer you can still see it's characters 0x02 and 0x17 if you look at the corresponding location on the left side.
Is there an actual application error being caused by DevTest, or is this just about DevTest not printing unprintable characters?
FYI: If you're leaving the response body as binary in the VSI when you run your VS then that disables most of the magic strings functionality, it will always send the same response verbatim. I just did a little testing and you should be able to switch your VSI response to text and it will preserve the control characters. You won't be able to see them, but if you save, reload, and switch back to binary then they will still be there.