The most likely cause is the indoor lighting, not a bad sensor.

Many LED stage/event lights, projectors, and some dimmed artificial lights don’t emit steady light. They flicker rapidly using electronic modulation (often PWM) for dimming or color control. Your camera sensor is read line-by-line rather than sampling the whole frame at exactly the same instant, so different rows can record slightly different brightness or color. That shows up as horizontal banding/stripes.

Why it appears in one image but not another:

• Indoor event lighting is a common source of this problem.
• A night-sky shot won’t have that flickering light source, so the pattern may disappear.
• Raising exposure in post makes subtle banding much more visible.
• High ISO can make the issue easier to notice, but it’s not necessarily the root cause.

So this pattern is consistent with flickering artificial lighting interacting with the camera’s readout, especially in venues with LED/DMX lighting gear.