As others have stated you have dark frame subtraction turned on. For astro shooting the general advise it to turn it off and if you want to do dark fram subtraction shoot your own dark frames (shoot a bunch) at the end of a session at the same ISO and shutter speed. There are tools that will combine the collected dark frames will using averaging to produce a master dark frame that will be subtracted. A master dark frame made from several darks will be a more accurate representation of the systematic error you are trying to remove from each light as the true random noise will be driven down substantally. This also better preservers the actual data in each of the light frames. I can go into a more detailed explanation if desired but won't at this point.
As far as the dotted lines/trails you are seeing in your final image that is because of the interval mode with additive selected. After each shot astrotracer will reset the sensor position and begin tracking the sky again so the framing is slightly different between each shot. Depending on what you are after the best solution is likely to just shoot a pile of light frames (shoot the darks while you pack up) and then combine them in post processing. There are a bunch of tools available here depending on what you are after. If it is just the night sky there are programs like Deep Sky Stacker (DSS), Astro Pixel Processor (APP), Pixel Insight (PI), Sequator, Siril, and probably others that I am unaware of. These will align images based off of star patterns and then either do some fancy match from simple averages to sigma clipped means and others, I also believe that some will also add the aligned frames together like what you are trying to do in camera. If it is a shot of the foreground landscape with the night sky then you really want to use Sequator as it has a feature to freeze the ground and also freeze the sky. Astrotracer can produce some great results even with huge lenses.
When I chase Deep Sky Objects (DSOs) I will use astrotracer with an intervolometer (functions like an interval mode shooting but likely with more options) but you are collecting individual light frames so not in camera averaging, addition, etc. After a few shots (depending on location and linear speed) I will reframe on the object and let the intervolometer take some more shots for me. I also shoot untracked with wides and ultra wides (photography wide and ultrawide astronomy used very different definitions) and use the rule of 200 for each shot (200/focal length= exposure time in seconds). There I will put the camera into the high speed continous drive mode and use a remote release cable with the button pressed (I made my own with a toggle switch) and just let the camera roll. Then I combine all the images in post processing.
A Deep Sky Object shot with my 400mm lens, K-3, and O-GPS1 for astrotracer with almost 9 hours worth of shots:
A night landscape shot taken untracked with my 12mm lens, K-3ii, Vivitar 285HV speed light at 1/16 power and diffuser with about 10 minutes of shots: