I actually found the answer now. All was ok, but the TestBase still needs to reset the Recorder of the inherited RulesTestFixture class. The Recorder counds the amount of invocations for the rule and is also used as reference when verifying the rules execution count using x.Rule().Fired(). So the Setup method of the TestBase must look like this:

    [SetUp]
    public void SetUp()
    {
        Recorder.Clear();
    }