Contact batch update via profile removes group contacts

Currently the contact batch update via profile is not usable, because it removes group contacts even if you do not uncheck any selected groups. In other words: Although not changing anything, "update contacts" removes contacts from certain groups.

You can reproduce this behavior on the drupal demo website (see my screenshots):

1. List contacts from any group (e.g. "group x"), select them and choose batch update via profile
2. Don't change anything
3. Click on "Update contacts"
4. Check the contacts details