Second time a valve broke, any idea what could have caused this?

Hi all,

First of all, let me introduce myself shortly,
I'm Ewoud, 23, and the proud owner of a 0.7L copen for a few years now.
I live in the Netherlands, and use my car as a student for all kinds of trips/ holidays/ work, but mainly, chilling and drive without roof

However, i have some engine troubles...

In april 2017, i have had a complete compression loss in cilinder 3.
The spark plug was broken, and both the curved bottem part which creates the spark as the 3cm "nail" part inside the sparkplug were broken, and missing.
This resulted in a broken exhaust valves, and a broken turbo.


We have replaced both the exhaust valves of the 3 cilinder(including oil seals), and the turbo.

Everything was fine, compression was 10.4 ; 10.2; 10.8 ; 10.6, and in runed smoothly.

Now the sad part, a few weeks ago in south of france, the third cilinder broke again after approx 1000km..
no compression, and after we (again) took the engine apart, we saw the same problem, a broken valve:


We can repair the valve again, but I don;t know what the underlaying problem is, that causes the valve to break.

Does anyone of you have any idea, what could cause the second time for the valve to break?

Kinds/regards, Ewoud