I don’t want to make any accusations, but I have worked at car dealerships. Let’s just say that when a mechanic was given your car to diagnose, she/he possibly could not replicate the complaint.
However, I have seen situations that occurred because of how warranty repair work pays/compensates mechanics for diagnosing and fixing warranty problems. Some manufacturers pay a limited amount of money for this warranty work (compared with work they do that is paid by customers) and sometimes mechanics aren’t highly motivated to do the work, let’s say. Sometimes they conveniently don’t hear or notice anything that would lead to them being “tied up” doing some warranty work.
Here’s what I’d do… Get hold of the Service Director/Manager at the dealership and discuss your written copy of the repair order you kept, showing your complaint and the mechanic’s diagnosis of “No problem found.” Arrange to take that director/manager for a ride, long enough to demonstrate what you’re experiencing and be sure they hear it and agree there’s a problem. Oh yah, be sure all your service complaints are written up and keep all receipts.
If this doesn’t help then try a different Hyundai dealership.