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.
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.