See this post on artifacts around Tethys - I think the cause of your strange shadow is the same.
I see this all the time with Jupiter's moons, except for the shadows it appears dark instead of light. Always the same triangular pattern, but often with uneven brightness of the vertices of the triangle. Someone with a better understanding of optics than me would have to explain it. It's not a processing artifact, though, as it can be seen onscreen if you increase the exposure enough.
Yes, the triangular pattern was fleetingly visible near the moon in real time, therefore not due to stacking or wavelet artefacts. It is to be expected that this effect would be integrated (summed) during stacking and appear almost to be something real. What is a little puzzling is that there appears to be a matching shadow. The shadow itself was much less visible in real time and its shape was not discernible.
I did wonder about collimation, but it looked reasonable during initial focusing. I will look more closely next time.
I set out with intention of capturing a clear, detailed image as per post #3, with a sharp, black circular shadow, but it just wasn't to be on the night.
Thanks for the link, I will read it with interest.