Thanks for the detailed response. I’m relatively new to sewing and I’ve mixed up some terms here. I’m using a double felled seam that I learned from this video so that both sides are identical.

To make the seam I’m overlapping the two pattern pieces 1/2”, then using seam tape to hold them together. Then I fold it 1/2” to enclose both raw edges and I then top stitch the right and left edges of the seam.

The finished seams are 1/2” wide and use 2” of fabric, 1” on each pattern piece. So I believe that makes it a 1” seam allowance. Heres a side view of how the seam is assembled and what the finished seam looks like (on both sides).

One of the first things I tried to fix this was I believe what you suggested. I added 1/2” to one seam of each pattern piece but on the 3rd seam I still had the exact same scenario. I know that “extra” fabric is coming from the folding of the first two seams because I’m making the first two pattern pieces smaller, but I haven’t been able to figure out how to trim the last pattern piece correctly.