Turns out when Cold War Shooters made the weapon CA-legal (by installing a "tool required" mag release), the new mag release was either improperly installed, missing a part or <insert problem here>, thus preventing the mag from seating properly. They recognized the symptom immediately, but would say exactly what the problem was.
I dropped the weapon off today for repair at their Highland, CA shop. Unfortunately, they're so slammed with backlog on new weapons, they couldn't fix it on the spot (a 10 minute fix, tops), so I'll have to take another 3 hour round trip by car to get it when it's done
Nice folks... but am rethinking where my next purchase might be due to this customer service issue.