For D6 nodes which are not in the default language (or language neutral) and are not translations of another node i.e. tnid = 0,
after a migration to D8, field values on the node are assigned the site's default language instead of the language of the node.
This means when viewing or editing the node the field's value is blank/empty even though the value is in the database
after the D6 to D8 migration.
Functionality can be restored on the Drupal 8 end by setting the langcode in the associated field and field revision tables to the same langcode
as the node so I guess something similar needs to happen at the time the field is processed in the migration.