A fellow forumer has had a different, but possibly related issue with the 2nd row passenger side seat. Perhaps it would be worth your while to check this thread out:

8 passenger 2nd row issue's not sliding | Page 2 | Toyota Sienna Forum - siennachat.com

 

Anyone figure out how to fix this issue?

 

I have completely removed the seat and inspected it in up Side down condition. I can see the mechanism that suppose to keep the seat in latch position but it’s just not working and I don’t see a way to fix it. I gave up and just keeping it as is. No big deal.