I'm seeing the exact same issue on my S25U. Therefore, I don't think it's an isolated issue.
I took some night mode shots on a cloudy night yesterday and didnt see the issue. But taking some more on a clear cloudless night like tonight, I see the banding.
It's got nothing to do with having a case or not (tried both and get the same results). I also don't have any after market lens protectors.
I'd encourage all S25U owners out there to try night mode in a dark environment and check for these artefacts.
I've submitted a bug report so will see what response I get.
Some samples attached:
