I have had a similar issue with my 2017 RX450h F-sport. It passed a recent MOT but the creaking noise was still there on the rear right hand side when I went over raised traffic control ramps. My local Lexus garage checked it out and replaced (yesterday) both L and R training arms, bolts and bushes as both were reportedly showing signs of wear. It has cured the problem.
The main issue however is that the bushes are not covered under the Lexus relax warranty and Lexus do not supply the bushes only so you have to buy the whole inclusive arm. I complained bitterly but the best my local garage could do was to reduce the labour costs as much as possible. I still had a bill for £1,100. Lexus turned down a warranty claim despite requests to reconsider from the garage service manager. The car has 54,000 miles, I have had 4 Lexus SUVs over the past 15 years and have never encountered this before.