Exposed Filter Filtering on Unpublished Revision
After figuring out the magical combination of keywords (it's "views filter exposed content revision") I found someone with the same problem:

"Unpublished data belongs to the latest revision of a content type is accessible for querying by the Views module"

https://www.drupal.org/node/2098211

Drupal user fbrooks replied:


  lavms,
  
  I don't think modifications to the Revisioning module are necessary to address this.
  
  I was dealing with, what I believe is, a similar issue. I have a content type with a boolean field used in a filter in a view such that a node will appear if the field if set to true. When you create a node revision under moderation that has the boolean field set to false, the node would be filtered out of the view before the revision is published.
  
  I was able to address this by recreating the view and selecting "Content revisions" instead of "Content" from the "Shows" drop down list on the new view page. This results in node_revision instead of node being used as the base table, and automatically adds a join between the node_revision and node table on vid. When selecting fields for display or filtering I used the ones that begin "Content (historical data)", which results in the values from the field_revision_* instead of field_data_* tables being used. All these things combined resulted in the correct revision being used by the filter.
  
  Hope this helps.
  
  Frank


This is the "core view" of the site and is quite entrenched so recreating it will not be simple, but it has turned from an unknown problem to a known and solvable one.

Thanks sareed for your help.