The passphrase is taken directly from the tty but the input tdata from stdin.
These are different input sources. The passphrase prompt pops up as soon as gpg
needs it.
You won't see the output on the tty becuase the sender used the
--for-your-eyes-only feature. Here is a trick to show it anyway:
gpg --output -
[Please send firther usage questions to the mailing list and not to the bug
tracker.]