Originally posted by MossyRocks: At 18mm the stars just may be smeared some in the corners from astrotracer. I haven't done any astro tracer shots wider than 50mm but I have heard others state that so that may be something to consider.
Yes, that is probable given the exposure length at 18mm on APS-C. However, in my experience that leaves the stars near edges and corners looking like trails, not multiple images. Both artifacts are likely present in this image.
AFAIK, DSS cannot properly handle the geometric distortion from wide-angle lenses when aligning the frames for stacking (although there might be a setting for that I am not aware of). The problem is the movement of the stars between images that happens with Astrotracer, even if you re-center between shots. That causes the lens's distortion to affect each frame differently, which in turn results in multiple star images where they don't register correctly. If the OP stacks with suitable geometric correction, there will likely still be trailing near the edges and corners, as you say, but it should be a big improvement.
FWIW, I have been able to do two minutes at 28mm on APS-C without objectionable trailing in the corners and one minute with none at all. For 18mm I have found that at 30 seconds the trailing in the corners is acceptable assuming no pixel peeping.