After getting home from an eight-segment mileage run (BOS-EWR-FLL-IAH-PHX; LAX-IAH-TPA-CLE-BOS), I saw that I had indeed received EUA emails on all of the segments for which I had been upgraded (7 out of 8--only IAH-TPA did not clear).

However, on the CLE-BOS leg, for which the EUA cleared three hours before the flight, the seat assignment read "01 SEA" rather than a more normal 01B or 03F (I was given 1B on this flight). Otherwise, though, everything else in the email looked completely normal.

Any clue as to what caused this quirk?