I have linked this phenomenon to a specific feature of certain sewing machines, This Kenmore 17881
cause the folded-over seams but the Singer 403 and the Pfaff 1222 that I have used recently do not.
I think I have the issue pinpointed to the configuration of the throat plates. The Kenmore and the Riccar have throat plates elevated a bit above the surrounding surface
while the Singer and Pfaff have throat plates level with the surrounding surface.
The elevated lip of the Kenmore and Riccar throat plates catches the seam allowance and pushes it back as the fabric feeds across the plate. When a seam approaches the throat plate, I have to lift the fabric up and physically push the seam allowance where it belongs, then continue sewing.
Now that I know that, I will not use those machines for projects involving sewing over seams.
Ed's Vintage Sewing Machine Shop