I was just reading the 57th Massachusetts regimental history, they do not flat out say they lost a flag at North Anna River. Lt. Col Chandler was mortally wounded and was known to have carried the flag last. When he fell several of his men tried to move him, but he told them to get out of there. Col Harris of the 12th Mississippi found Lt. Col Chandler and he died several hours later. But not a single mention of a flag lost.
Now the 57th Mass does admit to losing "colors" at the Crater. The 57th Mass was a front line regiment in the attack. The Crater battle has a few still unknown flags, hopefully they will be identified in the future.
Have a good day, everyone
Shawn Prouty