This may be a red herring, or a load of rubbish, or both, but according to this post: https://www.clublexus.com/forums/rx-4th-gen-2016-present/841435-heated-seats-and-steering-wheel-not-that-warm-16.html the heat of the seats depends on the temperature setting in the car.
Edit - if you scroll down to post 245, there is mention of someone having an amplifier changed.