Another user reported in this (I can verify it):

During a full refresh of the keyring, gpg seems to output all information
to STDERR and STDOUT. This makes it inconvenient to have a cron job to refresh
keys, because it can result in a very large and fairly useless mail.

Please ensure that normal output goes to STDOUT and errors and warnings to
STDERR so that problems aren't lost in the noise from this command.

Indeed some "normal" messages go to stderr and some warnings go to stdout.

I need to verify this. It is possible that we do a keylisting while importing
keys and the keylisting prints to stdout. If that is the case, we can't change
it because gpgme and scripts may reply on it.

Using --quiet for --refresh-keys makse sens, though.