If you use the command "bkuptocard " with gpg --edit-key the key from the "sk" backup file is not only copied to the connected smart card, it is copied to the private-key folder in the GNUPGHOME, too (protected-private-key instead of shadowed-private-key).

This makes it necessary to manually delete the corresponding key file after using the command, which is not user-friendly and could easily be forgotten.

In the gpg 2.5 branch (tested with gpd 5.0.2), bkuptocard moves the key to the card without a local copy left behind in the keyring.