It is super critical (so neither a liquid nor a vapor), and the decision to call it a liquid is rather unfortunate. I will check why this is. As a result of this, the density routine that is invoked is not that of the equation of state (which is the density routine for the vapor phase), but the default selected for the liquid phase, which is COSTALD, which is of course completely off.
I wouldn't call air at ambient conditions supercritical though; it is above the critical critcal T of -140C but well below critical p of 38 bar thus still a gas.