Well spotted Freddy!
In fact properties do not work on new checks:
Previously, checks were using getters and setters for Sonar properties, and no one knew exactly why this was working.
I simply removed those methods, and all seemed to work fine.
But in fact it does not, because the Web plugin had some logic to instantiate the checks itself (based on the profile), instead of relying on Sonar to do so.
It then used reflection to call the properties setter methods.
Long story short, this will take a bit of time to fix.