Comment 19 for bug 292606

"man 7 debconf" states-
       * Avoid outputting anything to stdout in your postinst, since that
              can confuse debconf, and postinst should not be verbose anyway.
              Output to stderr is ok, if you must.

It might be better to redirect stdout to file descriptor 3.
I see this comment in /usr/share/debconf/confmodule
        # Redirect standard output to standard error. This prevents common
        # mistakes by making all the output of the postinst or whatever
        # script is using this library not be parsed as confmodule commands.
        #
        # To actually send something to standard output, send it to fd 3.

But I don't see any documentation outside that file that directly says that fd 3 should be
used in the role of stdout.

The nvidia-common package certainly cannot be expected to make up for the problem that
the dkms package creates. The bad output is already sitting in a pipe headed to
the debconf frontend before the nvidia-common postinst script is ever run.