With GnuPG 2.1.21 (libgcrypt 1.7.6) on Windows 10 Pro 64-bit 1703 (OS Build 15063.413) gpg -d does not consistently prompt for a passphrase and expedites decryption if a file was encrypted with gpg -c.
On the other hand, gpg -d in GnuPG 1.4.21 prompts for a passphrase every time the command is executed.
Is this indeed a bug or is the symmetric key in GnuPG 2.1.21 cached somehow?