bschnitzer Those look like severe jpg compression artifacts, but I've never seen Seestar do anything like that.
I don't think it's environmental, because the scope re-frames between shots quite a lot during stacking (they call it dithering but it's larger jumps than normal dithering). What that means is anything that could cause this in front of the sensor would be immediately wiped out by further stacking. Not to mention that the sky has rotated several degrees during even 9 minutes of exposure (up to double that real time), so no way it makes perfectly crisp horizontal lines in multiple places in the image.
What it looks like to me, is a low quality preview image. I've seen this when someone on an iOS device shares images into Google Photos when they're not on wifi. It saves a full dimension but smaller file size image, and then later when they are back on wifi (if they remember to reconnect to Google Photos) it updates it with the original image at full res/full Quality. I don't know if it does the same thing if you stay inside the iOS universe, but that's what it looks like.
I notice your original jpg posted here (unstretched) is only 177KB, but when I look at my own stacks for IC 434, most are over 500KB, some as much as 700KB. Do you see the lines in the original image if you view it directly on your iOS device, or only computer later?
