I don’t believe it’s a bug, as your success rate is based on your score. Just like in the Scenarios you earn a score by how much you can rise above the minimum requirements. Getting a higher score will increase your success rate as well.
On a different note, I had a crash a few minutes ago, I created an NA 1L I3 engine, created a clone and turned the second one into a turbo. When I finished, I closed the editor and clicked the button to create a new trim. This is the screen that greeted me: