This has nothing to do with Specflow or Reqnroll - I think there is a bug with the safari driver as i believe it hardly gets updated on a regular basis like the chrome driver. The reason why i say this is because this has happened on previous versions of the chrome driver and after an update the issue was resolved. The Quit() method should always close the browser and kill the driver process regardless of the driver/browser being used.

Try it on chrome and then on safari.

If it works on a chrome but not on safari then its a bug in the safari driver.

You can create code to handle the bug by tracking the process id for the browser and kill it but this will just add a more convoluted mess of code for a bug you are not supposed to hack around. Plus the code will then become unnecessary once the bug is fixed.