At least as of 2017, the NEXUS computers were very disconnected from the systems regular CBSA agents could use. Before I got my Canadian Citizenship certificate (first generation born outside Canada) and my passport, I had a Canadian work permit under NAFTA. When it gets close to expiring, the NEXUS computers will automatically print IMM-1-1 and you will be sent to Immigration I. Theoretically this is to determine your overstay risk.

At Immigration I, at least three times the customs officers told me that they had no idea why I was referred and after a couple minutes chatting (after 10-30 minutes in the Imm 1 line) they stamped my US passport and sent me on my way.

Once I had my Canadian passport I went to the NEXUS office at YYZ and had them add my citizenship/passport, those CBSA officers said "oh, your work permit is flagging it, since you're a citizen you don't need a new work permit and we'll remove it so you aren't flagged".

I'm planning on renewing my Canadian passport early (due to changes in physical appearance), when I do I will probably use the web form. If I get a response email back that is a match for email authenticity (SPF/DKIM/DMARC) and used encryption during transit, I'll probably just send the requisite information back over email as I do not live close to an enrollment center.

Point being, at least historically, the NEXUS computers are relatively an island. Regular CBSA officers don't see the NEXUS information at checkpoints other than the kiosk printout (at least, again, as of 2017 at airport points of entry), and when I added my Canadian citizenship/passport at the YYZ NEXUS enrollment center, the US CBP officers added it first, then handed it over to the CBSA officers to enter second. So it sounds like not only is the NEXUS system/frontend application not accessed frequently by most CBSA officers, but that it's very disconnected and manual between the US and Canada as well.