My car started shaking when idle at 12,000 miles. Is this normal
No, if your service department says it is normal, try a different dealership. Get it fixed under your warranty.
Is the check-engine light on?
No, the Check Engine light never came on
Have you taken this to the dealer because you have a warranty . If you try to do something to it they might not be willing to solve your problem. Your header does say 2018 is that correct?
No, not normal. Something’s wrong. Probably something minor, but might be difficult to diagnose. Good advice above to just take it back to the dealership for a warranty inspection and repair. About the only thing a driver could do that would contribute to this at 12,000 miles is getting a tankful of bad gasoline. Did you refuel at a gas station you don’t normally use? Maybe one in a small town that doesn’t sell much gasoline?