Gpgtools GPG Keychain Access

I have a YubiKey 4 Nano that contains subkeys of my master key. My master secret key is kept offline and the private keys for the subkeys are only on the smartcard, not on my laptop.

When the YubiKey is NOT plugged in and I select some text in a text editor and try to sign it using the "Services -> OpenPGP: Sign Selection" menu, the text is transformed to have the "BEGIN PGP SIGNED MESSAGE" header, but no signature is present. Instead, I should receive an error that the smart card is not present.

When I try to encrypt text using "Services -> OpenPGP: Encrypt Selection", the encryption seems to happen flawlessly, except that obviously it was not encrypted with the correct secret key since the YubiKey was not plugged in. It seems that the message gets correctly encrypted with the recipient's public key, but cannot again be signed with my private key.