Resolved. The problem was much simpler. We had a quotation mark in an incorrect location in the mashup shortcode.
Was:
[mashup query="post_type=preserve" width="100%" zoom="7" maptypeid="preserves1"]
Should have been:
[mashup query=post_type="preserve" width="100%" zoom="7" maptypeid="preserves1”]
Working. Case closed 🙂