Need to see more examples in different conditions with EXIF included and what shutter mode you were using (electronic, first curtain, mechanical). Would also love to see the same photo take with lower ISO settings and different shutter speed.
Knowing nothing other than what I see in front of me I can posit a lot of possibilities, including an ultra high rate of light flicker that is captured perfectly by the sensor sweep at this specific shutter speed, the same sweep responsible for rolling shutter in other types of shots. If you're only seeing it in this conditions at a particular shutter speed I suspect that's more your problem than low light and high ISO, but more information and examples will tell the tale.