Ok, back in front of KiCad. The reason the missing GND net connection wasn’t flagged was because I had added a filled GND zone around the components, and the ESD protection diode got connected to the zone.
(Btw, the two DRC flags you see are warnings that the pads on the USB-C footprint are too close, but this is coming from ai03’s lib so I’m going to assume that the footprint is ok (famous last words…)).