When you're first shown as sitting in 1A, that's only an offer not an allocation. You have to confirm it, and it is not allocated unless it shows on the main MMB page.

I'm not quite sure why you aren't able to confirm the seat straight away, and I haven't got any new bookings to test this on at the moment. But what I suggest is clicking on the icon in 1A and seeing whether that activates the confirmation button.

If it doesn't, try this: move to another seat, click confirm, and then immediately move back to 1A and click confirm again. I think that this sequence works without having to leave the seat selection page.