I am using dynamic axis on job that can run concurrently.
The plugin evaluate axis by on of the job's parameters.
If (and only if) this parameter is empty, the plugin should set one value to the axis called "dafault".
The issue is that sometimes even when this parameter is not empty, the plugin adds the value "default" to the axis, causing undefined runs to the build.
From initial look at the code, I suggest it uses Java collection that doesn't support concurrent runs, not synchronized.
Found another thread on it on the web.