I read an old post on here that the 7th Michigan lost a flag at Sharpsburg. I have not personally found anything on Union flags captured at Sharpsburg, besides the 103rd New York lost their regimental flag to the 17th Virginia.
I do question this Union regiments that were in Sedgewick's division and were hit hard and took many casualties.
59th New York, 15th Massachusetts, 42nd New York and the 7th Michigan. It seems that Howard's all Pennsylvania brigade also took a pounding and retreated back a bit.
Early's brigade hit the 59th New York right on the left flank. Most of the Union regiment in that area, wrote how they were hit from the front, left flank and rear.
There as got to be more then just 1 Union flag captured/lost at Sharpsburg.
Even when I look these regiment's history at the LOC, not much is said about the flags.
Many Union regimental commander were embarrassed by the loss of a flag and most of the time, never reported it on their after battle report. Then I will read what the Confederates wrote and somewhere in the middle is the truth.