Hm, yepp. I can see it as well.
This worked just fine,
mender-artifact write rootfs-image -t beaglebone -n mender-2.0.1 -f core-image-base-beaglebone-yocto-grub.ext4 -k private.key -o artifact-signed.mender
But trying to sign an existing artifact gave me a similar error to yours,
$ mender-artifact sign artifact.mender -k private.key -o artifact-signed.mender
Could not finalize tar archive: archive/tar: missed writing 37277864 bytes