ANSV says the tiltrotor’s aerodynamic behaviour at high speed was not accurately predicted by the manufacturer Leonardo Helicopters; during simulator tests it proved virtually impossible to replicate the accident sequence.
From your second linked article. (Commando Cody)
The limitations on simulators and simulations is that you still have to find out what the aircraft actually does when you are establishing the performance envelope that your customers can use.
(For an example I am familiar with (my brother in law's dad worked for Beach ages ago) go back 40 years and check out what happened to a T-34C as they were working their test program up toward the 300 knot spec. The tail failed in flight. When the Navy finally put it into service, the red line was 280.)