I'm booked to fly ET from LHR to FCO on a 21A. On the 'select a seat' seating plan, ET starts at row 4 but seat 4B is missing. It's not blocked out or already reserved, the actual seat icon is not visible. Seats 4AC and 4DEF are available. On the return leg, ET starts at row 7 with the same type scenario (7B not visible).

Anyone any ideas why? I've looked around but can find no reference to it.

Thanks.