I have seen a pattern to this.

Often the furthest forward ET row in MMB will be around row 8; then on the Sunday(?) they move the curtain to row 4 and you can grab that; however the curtain is still in movement and gets moved back to after row 4 and you are stuck in limbo in MMB, unable to view your seat nor change it.

What happens to me is that 24 hours later the curtain gets frozen and if the seat shifters are kind they will give you row 5 but if not you need to be pro-active!