As was discussed in a very recent thread (like, yesterday), the reservations system and the departure management system are not the same... when you change your seat via OLCI (or when it is changed by Operations, as appears to be the case here), it will not be reflected in My Itineraries, which pulls data from the reservations system.

I can't speak to whether your seat change is real or not - I guess you'll find out tomorrow - but it isn't surprising that OLCI and My Itineraries don't agree, and that isn't an indication of a problem in and of itself.