Earlier quoted context omitted.
I also just tested now and reproduced the bug. The key thing is that you are pasting back into the calculator – which presumably is just stripping letters. The behavior is maybe a bit surprising, but I could also see it being defensible. You can't type "1e20" into the calculator, so why would you be able to paste it in? Outside of plain text, I don't think clipboard operations are necessarily expected to be reversibl…
That’s a good point. Maybe you should be able to type “1e20” in scientific mode? It would make the calculator more feature full and prevent “losing” data if you copy/paste in the middle of a long calculation.
That said, I tested again, and once again copying "1e20" and pasting it into the Calculator works just fine. It's definitely not treating it the same as pressing each key separately. I'm testing on macOS 13.0.1.