This is just a simple Glass BSDF (but different glass shaders act the same way).
I cannot replicate the issue on another blend file.
Tweaking with SubD, smooth shading, light paths didn’t do anything.
It’s driving me crazy trying to isolate the issue. Does anyone know what might have caused this?
After your testing, it seems your blend file has some issues.
What you could try, is to start with a new file, and start appending parts of the file that is giving you issues.
At some point, the render error should show up again. And that’s your culprit right there.
I know, a pain in the behind, but sometimes things like these are necessary to figure out stuff