I would be far more concerned about the paint cracking than the seam. Any idea what type paint was used? It sounds to me like they either used an inferior filler to fill the seam and it is expanding/shrinking at a rate different from the surrounding area or that they used an incompatible paint/filler combination. If the paint was base coat/clear coat then they will need to sand out the bad area, re-shoot with base coat, then clearcoat the entire panel--in this case the entire rear fender. If they used a single stage paint then they might have a tough time blending it in to look right. Either way, I suggest you take a very close look at all the seam-type areas for any other evidence of cracking. My guess on the seam is that they simply covered it with filler, in which case it can easily be fixed.