DCT or manual? If it's the DCT then it could be the DCT was misbehaving (too high a gear for the power given) and causing your engine to knock/misfire. If it's the manual, no idea and I don't have any technical suggestions *but* if you live close to another dealership, but if you left and the issue persists you might consider taking it to a different dealership's service department instead.
I purchased the 2019 Hyundai Elantra GT N Line turbo / automatic...car new in August 2019 and today Oct 16th I took car in for oil change have 3500 miles on it. I left and everything was fine...drove highway speeds 70 mph using cruise control for 2 hrs. On return trip I tried to pass a semi and I had no power ..no acceleration.. rpms went up a little but no full acceleration/no power ... I was able to still drive .. but could not accelerate/ no power ... felt like the turbo had gone out. Took to dealership that I had gotten the oil change earlier that morning..didnt think it had anything to do with oil change .. they looked at it for about 3 hours .. they checked it for issue/fault codes found that it had several misfires.. So they swapped 2 of the ignition coils around and 2 of the plugs ..until they had no fault codes ..( but prior to them doing that I wanted them to keep car running so they could test drive , but a service advisor turned it off & the issue I had went away) also I noticed the exhaust sound ..sounded rough .. not smooth as in previous days/ months ) I picked up vechicle and all they had done was swap 2 ignition coils and 2 spark plugs .... they did not replace with new ignition coils or spark plugs ... Does any one know if there could be any other reasons for what had happened.... please advise ... I'm concerned that they just patched the issue ...& didnt fix the issue since they acted like they didnt really know what had caused the problem.... Thanks