I can't reproduce what you see, so far as the seat maps are concerned, when searching without the discounts you're using.

So if you are still seeing the premium economy cabin (laid out 2-4-2, as Cymro says) for your IAD-LHR flight when you have selected and are being quoted for economy in that direction, my money would be on glitch. If you successfully pick seats there, I wouldn't expect them to stick - you'd get lucky if they did.