The IAS in the sim does not correlate with TAS and Mach number at high speeds.
This bug is not my finding, but was discovered by the FlyByWire team, that since then fixed it for their A32N mod.
Here is a link to their github about this specific problem. Included there are documents showing the formula that should be used to calculate the IAS/CAS (same thing for aircraft with an Air Data Computer) from the TAS : https://github.com/flybywiresim/a32nx/pull/2745
However, this bug is affecting all the aircraft flying at higher speeds and has potential repercussions on the flight model accuracy and reporting as the IAS displayed in the sim are wrong.
Here is a screenshot taken in the 747 showing level flight at 310 KIAS and FL300, notice the IAS, the TAS and Mach number, ISA conditions :
Here is a screenshot from an online calculator for TAS/CAS/Mach for the same conditions, notice that for the same IAS, TAS and Mach are lower, showing that the IAS is over-reading in the sim :
