Might be a defective scrim that sometimes has a bump that might not be noticable.

Steam?

It’s not defective foam, I think it’s due to the fact that the sew foam is rigid and when it wraps around the edge it props the middle up. I was considering next time sewing/gluing a plastic piping behind the top stitch and pulling it back through the foam like a pull bar but less prominent

Did you take the pattern from the old cover? Some times this happens if new cover is a bit bigger.

You could always use velcro(loop) sew foam and glue a strip of the velcro(hook )to the cushion

Good tip Rob. I will use that for the next set.