Thanks for the feedback.
Please be patient : it will be long and difficult to solve, but we will find it.
The main reason is that this "json diagram" feature is the first official use of the smetana project.
Smetana is a source translation of C Dot/GraphViz to Java. This means that "json diagram" don't use any external GraphViz installation. It runs in pure Java.
So this bug means we have done a slight mistake somewhere in our C-to-Java translation. To find it will not be an easy task...