Luckily the trifold liner seems to be enough weight to keep the seat flat and from popping up. I have given up on Toyota solving the problem at this point. Both Toyota customer service and the local dealer have said this is normal. They have a very weird definition of normal. I guess thats the risk of buying a first year model.Unfortunately I also bought the 2021 sienna. I hit a dip in the road and the the third row seat came flying forward and latched with the loudest bang I ever heard driving a car. I thought I was in a accident. I took it to Toyota Escondido in San Diego and they said nothing they can do. I'm glad my dog was not back there . He would either dead or had a broken leg. The Toyota dealer said to put some weight on the seats when down. (Real smart, just don't hit the breaks or you will have weights go flying through the windshield.) I had a 2014 Sienna and it did not have this issue.