There is a little bit more resistance of roll-back force when the wheels are lowered vs. when they are all the way up. When all four wheels are raised to the highest setting, the pull-back force should measure 13 lbs. or less. (You really need a scale to be sure.) If the force is more than 13 lbs., the SmartDrive cable may need adjustment. You can also double-check to make sure the control arm spring on the transmission is okay (not broken or missing), and double check the belt guide clearance on both the crankshaft and transmission pulley are set correct, about 3-4mm and 2-3mm respective. If all those are fine, removal of the rear wheels and some application of general purpose grease to the adjuster arms is recommended. All of these steps are outlined in the shop manual for the HRR216VKA.
Now of course all this is just FYI, as you have a mower that is still under warranty, so I'd suggest you just take it to a servicing Honda dealer so THEY can do all the above. Just be sure to confirm with them not to do any billable work before they get started. Understand
some dealers charge a "diagnostic fee" up front, but if the problem is shown to be a factory defect and covered under warranty, the fee will be eliminated.
Here's where to find a Honda dealer in your area; be sure to click on SERVICE and LAWN MOWERS and then your ZIP code:
Find A Honda Dealer