Yes—this is typically caused by the camera changing exposure between frames. For timelapse, even tiny frame-to-frame changes in shutter speed, aperture, or ISO show up as visible flicker in the final video.

To avoid it, shoot with fully manual settings:

• manual aperture
• manual shutter speed
• manual ISO
• disable any other automatic exposure adjustments

If even one setting is left on auto, the camera may compensate differently on each shot, causing brightness fluctuations. One community answer specifically noted that Auto ISO alone was enough to create flicker.

For a sunset sequence, the tradeoff is that fully manual settings keep the timelapse smooth but the scene will naturally get darker over time. If you want the exposure to change smoothly as light fades, that requires carefully controlled manual adjustments rather than letting the camera recalculate each frame.

If the images are already shot with auto exposure, your options in post are limited. You may be able to reduce flicker somewhat by matching exposure between frames during editing, but the best fix is preventing it at capture time with full manual control.