I have been having this exact issue with my 2016 4runner with 7500 km's on it. It started doing this a couple months ago, I took it in to one of my local Toyota dealerships (Country Hills Toyota in Calgary AB) and they said it was "normal" and they all do this. I then brought the issue up with Toyota Canada and they arranged for me to take it into another dealer. I took it into Charlesglen Toyota this past Tuesday and the service manager ordered a new complete seat frame that they are going to install to try and get rid of the issue. I am a little worried now after reading this thread that this will not solve the problem since you guys are experiencing the same issue as well.