Needless to say, this is quite aggravating, especially since immediately
following hitting <enter> the packages install without interaction.

I just figured out a bandaid type solution to the problem, which works around
the issue by altering the command line like this:

pkgadd -d . < /tmp/all.txt
pkgadd -a ./responses/noask_pkgadd -d ./HPNP -r ./responses < /tmp/all.txt

The file all.txt consists of one line and the enter key, with the word "all"