This is likely not true scene moiré, but an artifact caused by noise plus downsampling/sharpening during export. At high ISO long exposures, the sensor noise pattern can become visible, and resizing to 640×640 can turn that noise into repeating patterns.

From the replies, a few things can help:

• In Lightroom, try turning off Enable Profile Corrections. One user found this removed the artifact.
• Reduce or disable sharpening on export.
• Apply some noise reduction before resizing.
• Export larger first, then resize in another app to see if Lightroom’s resampling is the trigger.
• For cleaner files, use high ISO only to test exposure, then shoot a longer exposure at a much lower ISO. That generally produces much cleaner skies and fewer artifacts.

So the issue is most likely the combination of high-ISO noise and Lightroom’s correction/resizing pipeline, not a fault with the lens or camera.