TYPO3 Core

Assume we're having the following form. Steps to reproduce the problem:

1. select one element of checkboxgroup
2. submit form
3. expected: selected checkbox is submitted
4. is: all checkboxes are selected

prefix = tx_form
confirmation = 1
postProcessor {
    1 = mail
    1 {
        recipientEmail = your@mail.com
        senderEmail = your@mail.com
    }
}
10 = CHECKBOXGROUP
10 {
    class = fieldset-subgroup
    legend {
        value = Select me
    }
    name = checkboxgroup
    10 = CHECKBOX
    10 {
        type = checkbox
        label {
            value = Option 1
        }
    }
    20 = CHECKBOX
    20 {
        type = checkbox
        label {
            value = Option 2
        }
    }
    30 = CHECKBOX
    30 {
        type = checkbox
        label {
            value = Option 3
        }
    }
}
20 = TEXTLINE
20 {
    type = text
    name = surname
    label {
        value = Surname
    }
}
30 = SUBMIT
30 {
    type = submit
    name = 6
    value = Submit form
}
rules {
    1 = required
    1 {
        breakOnError = 0
        showMessage = 1
        message = Required
        error = This field is required
        element = surname
    }
}