I sent an email to the webmaster of Union Corps histories. He has the 12th Corps history as never losing a flag ever in its history from September,1862-April,1864. As you all know the 11th Corps and 12th Corps became the 20th Corps. That ended the 12th Corps.
33 days before the 12th Corps formed, at the battle of Cedar Mountain "Army of Virginia" Bank's Corps, A.S. William's Division, S.W. Crawford's Brigade. The 5th Connecticut and 28th New York lost flags. 33 days later they are put in the 12th Corps and legend has it, the corps never lost a flag. This brigade had the highest losses at Cedar Mountain.
5th Connecticut--145 Captured, 21 Killed, 71 Wounded=237 Loss
28th New York-----113 Captured, 21 Killed, 79 Wounded=213 ""
46th Pennsylvania-111 Captured, 31 Killed, 102 Wounded=244 ""
10th Maine----------4 Captured, 24 Killed, 145 Wounded=173 ""
The 5th Virginia took a flag from the 5th Connecticut and the 28th New York.
Have a good night,