I had the same on a recent trip. My partner and I were moved 2-3 times from our selected seats to other seats - I then put us in more seats that we preferred, etc. Given EP status for one of the passengers on the reservation, I was surprised to be moved out of MCE seating to MC seating (from row 11 to row 13 on a 788). Fortunately the flight was not too full, so we had 3 seats for two people, but I would have been far less pleased if I had had only one seat without the extra legroom. My legs don't fit into the standard 787 pitch very well.

It's the sort of thing that makes me want to put an alert on my own seat so I can see when I've been moved out of it!