I have talked with my dealer, and they have an idea that the cap of the expansion vase is opening
Only when the pressure is too high , and then damage the radiator.
They think that the cars are tested on a racetrack , with an agressive drive,
The coolant and the pressure in the radiator increase and the cap open at that pressure.
As we dont drive so fast the cap stay closed but the pressure
Damage the radiator.Now they have placed a cap with lower pressure values.
Not easy to explain in english ,sorry
I recept a radiator cap from my dealer. The release is done at a much lower pressure.
I also had to remove the equivalent of a glass of coolant to reach 7 cm in the jar.