PJ2;
The 300 ft drop in indicated altitude is explained in IR#2, para. 1.6.11.6 Consequences of a drop in the measured total pressure (p.49). At cruise AoA the pressure at the static port is slightly higher than the true ambient pressure. The measured static pressure is therefore corrected by a term that is proportional to airspeed-squared (or Mach^2). That correction is essentially lost when the pitot pressure drops to a low value.