Same thing happened to me a few weeks ago, CE cabin was extended and absorbed the row I had previously selected an ET seat in. This seems to cause the system to say your confirmed window/aisle seat is a middle seat, maybe it defaults to that when your seat doesn't exist in the cabin anymore?

The thing that actually made me double check the seat selection was when I noticed the description had randomly changed from window to middle but the seat number (and aircraft type) was the same so it made no sense at all. That made me click 'change' to look at the seat map where I found the row wasn't on the seat map anymore (despite the page before still showing my seat as confirmed).

I was able to move into row 7 to avoid being reallocated into a real middle seat down the back at check in!