It's happened to me as well, and unlike OP I was put into non-MCE seats, and by the time I got the notification and checked - well, no MCE left (bar exit row, which I can't grab with a 7 y.o. in my family.

And yeah, the change in question was a 7 minute departure time change. Same flight number. No change in equipment (738, so no question of pmUS/pmAA 3xx switching). I was hopping mad - and eventually got them to put me on another (earlier flight) that had some non-exit row MCE left.