can't copy the key on the cursor in 2.82
I selected the bone and pressed control C and it shows an error.
And when I select the bone and the all the keyframes like you did it just copied all keyframes.
what I did was execute the "sample keyframes" command in the key menu and just move the generated breakdown keys and deleted the rest, but this took extra time. I wonder how you did yours with just using the copy and paste command.