Yes, you’ll see the seam in the fascia. There really isn’t a way to get around it. Fluted or Reeded fascia seems to make the seam less noticeable; probably because your eye focuses on the horizontal flutes, not the seam. And, solid, darker finishes tend to make the seam appear less pronounced. For wood fascia, furniture touch-up sticks can be used during install to blend in the seam.
For the most part, it really depends on the brand of Decorative Traverse Rod. Some brands use the term fluted while some use the term reeded. Typically, with fluted fascia, the grooves are spaced further apart. And reeded drapery rods have a “bumpy” appearance since the grooves are close together. Typically, all 1 3/8” diameter fluted fascia is “reeded”, even if it’s referred to as fluted. It’s best to pay attention to the example images and not the terminology.
For the rear track on a Decorative Traverse Curtain Rod, there’s room for unaltered 1 3/8” fascia on the back, but larger diameter fascia, like 2”, needs to be cut down. It looks odd from the side, but appears perfectly natural when seen from the front. Your finials or end caps will conceal the alterations when viewed from the side.