I can think of two possible reasons....
- The EDI/LHR sectors are causing this due to the removal of the Business Class cabin. I suspect that it's a glitch that'll fix itself considering you're on a business class award ticket. It's already been confirmed that bookings in "I" class will still entitle you to a seat at the front in the new 'Flexible Economy" cabin, hence your 2A/B will be safe.
or.....
- Your LHR-IAD is on United? and has changed from a 777 to a 767, which has no row 12.