> Thank you for contacting us with your inquiry.
>
> The text of our Targum Onkelos is from Chumash Toras Elokim (Vilna, 1874) which includes R' Wolf Heidenheim’s text of the Targum; that text is based on manuscripts and is considered מדוייק.
>
> We appreciate the opportunity to clarify this.

Cover page image from a later email from Artscroll.  It's not exactly the one @DoubleAA links to in the comments, pagination is also different, but the text of the Targum looks the same from a few pages they also included in the email.
[![cover page][1]][1]

I'm not marking this answer as correct because it doesn't explain the underlying reasons why Nachshon is different than the others.  In a followup email, they told me that they don't know any reason for the difference.

Maybe I'll try asking Dirshu when I get a chance.

  [1]: https://i.sstatic.net/5JkyD.png