Another possibility is that instead of the issue actually missing the publication date, there may be an element which is incorrectly occurring before the publication date. For example, perhaps the contributors element is malformed.
A sane formulation would be:
<journal_issue>
<publication_date media_type="online">
<month>11</month>
<day>20</day>
<year>2015</year>
</publication_date>
<journal_volume>
<volume>7</volume>
</journal_volume>
<issue>2</issue>
</journal_issue>
What does the journal_issue element look like for you?