I have also noticed as recently as yesterday that as a DM booking a Y seat, all of the "regular" Y seats were showing as an "X." Basically I could only select an exit row or other preferred seat close to the front. This was my first time booking main cabin in quite a while, so was surprised to see this. I selected preferred seats towards the front and was auto upgraded to C+ shortly thereafter. I then used miles to FCM to F and now all seats except the F cabin show as an "X."