I'm wondering if there was an issue with the software inside the card scanner or the supporting application.
Software updates will mess things up.
When I went to the CA DMV to get RealID, their card scanner couldn't read the U.S. Passport Card I presented to them so they asked if I had anything else, I had to ask the representative to allow me to return home to get the book version. Luckily I lived close and knew where my book was stashed so I was able return and get the entire thing done within 45 min. I suspected that my U.S. Passport Card was too glossy so the scanner (or the software) couldn't read it, which was mailed to me a few months earlier due to the laminate with picture and personal information peeling off the card. And all this time I was watching them scan my U.S. Passport Card.

I never carry my US Passport book or card with me inside the US. I used to carry the passport card with me, but it ceased to be useful to me anymore as GE can do the same thing (and more) and no longer need it to enter Mexico.
That and as U.S. Citizen I'm not required to carry these documents, and if questioned at a Border Patrol checkpoint, I can easily answer this question.