2015 Sonata SE has P0011 code. RPMs increase at idle/loss of power and mpg

Problem is not constant, usually after 30 minutes or more of driving. Car has 108,000 miles. Took to Hyundai dealer (Friendship in Bristol, VA) and had diagnostic run. Tech decided to replace the cam phaser cover assembly (1 week later, same code again). I did not have the leaking seal problem another TSB was sent out for. I'm set to return to dealer next week for re-diagnosis. I believe it's the cam-phaser assembly or an oil flow problem. This new design on the 2015 model makes this a much harder/more expensive repair if it is the phaser assembly. I have always changed my own oil and have used a full synthetic, changing the oil every 5K miles and using only Hyundai oil filters. I'm getting the song and dance from the dealer that Hyundai won't cover anything under warranty if I can't show proof of maintenance. Does anyone know if Hyundai can do this arbitrarily, or do they have to show physical evidence of lack of maintenance, i.e. sludge? If this is a failed cam phaser assembly, which technically is a cam gear, would it normally be covered under the drivetrain warranty? The warranty says timing chain and gears are covered. My warranty was extended to 120K under the class action law suit. I have seen many other complaints of similar nature online, and it seems the dealerships don't have a clue what's wrong with this model.