we have the same problem in our 450RX and it’s been back 4 times to the Lexus dealers it’s only 5 years old.
they are saying it’s the amount of water when we put the hosepipe on and the slope of our drive, which is not steep, it’s a small slope.
It rained all day Saturday night and Sunday morning the towel we put on the floor was soaking.
back with a them and they said there’s no blockage with pipes. The rails on top of car have never been removed. No roof box ever used. We are at a loss and it’s extremely frustrating.
how are you getting on?
oh, it only happens when the front of car is parked facing down the slope!