I assume this was taken with the included RACI diagonal. Bright stars/planets will likely show some longitudinal blurring in line with the diagonal, usually as single line. The diagonal might not be well aligned either, something I have seen in about half of the cheap achros. As has been stated before...the accessories are bottom of the barrel, so you can't really trust them to evaluate the scope's focuser or optics unless everything happens to look great together. If there are problems you have to try to eliminate various sources.
It could be inherent to the eyepiece if the problem was not happening with the higher power one (usually such errors go the other way, they become more noticeable with magnification.) Does the angle of the artifact change when rotating only the eyepiece?
If you are seeing the same thing with eye and camera then it rules out astigmatism in your own eye. By 3.6mm exit pupil (the 18mm eyepiece in an f/5) even fairly minor astigmatism can be noticeable. If the extension rotates when you turn your head to a different angle, then it is your eye.