I do pretty much the same thing. I put the LF2 sitting pose to fram 12 or 15 but in same time of the seat final position. There
is no other clothing, the dress that I am simulating is
"40ParisDress2-1" and it is not checked in the simulation's collision
parameters of course.
It's not a problem with the bench or the clothing since I had the same problem with another seat and another clothing previously. This is the 2nd time that I have done a seating pose with pose 13 and it is the 2nd time that I have obtained this result.
Have you try this in poser 13 ?