The seat shuffle just happened to me once again today. On the final payment page with credit card details everything looks good and my chosen seat is showed under "optional services" (valitut lisäpalvelut). However, as I press purchase, the confirmation page that comes up next has given me a completely different seat. I wonder if the same happens with paid seats as well, as for status passengers it is still relatively easy to just select the seat again. Definitely a big nuisance though!