Its a known issue, you should get this done under warranty. According to the technical service bulletin L-SB-0038-18: Some 2016 to 2019 model year RX350 and RX450h vehicles may exhibit a condition where there is a noticeable seat heater temperature difference between the driver and/or passenger seats. Follow the Repair Procedure in this bulletin to address this condition.
You can read the full service bulletin here:
https://static.nhtsa.gov/odi/tsbs/2018/MC-10152563-9999.pdf
Manufacturers often issue service bulletins or recalls to address known issues, and in many cases, the repairs are covered under warranty. If your vehicle is still within the warranty period and is affected by the described condition, the dealership should be able to perform the necessary repairs at no cost to you.
It's recommended to reach out to your nearest Lexus dealership, provide them with the information you have about the NHTSA bulletin, and schedule an appointment for the required repairs. They will have the necessary tools and expertise to address the seat heater temperature difference issue in accordance with the repair procedure outlined in the bulletin.
