"Missing field in target work item' warning despite having a field to field map config

[webturner]

I'm getting the following warning in the log:

[10:25:19 WRN] [v15.0.3] PopulateWorkItem:FieldUpdate: Missing field in target workitem, Source WorkItemId: 143341, Field: RSM.DevNotes, Value: <div>...

even though I've added a field map like so

  "FieldMaps": [
    {
      "$type": "FieldtoFieldMapConfig",
      "WorkItemTypeName": "*",
      "sourceField": "RSM.DevNotes",
      "targetField": "Custom.DevNotes",
      "defaultValue": ""
    }
  ],

The custom field in my target system isn't being populated.

Any clues to what I'm doing wrong?

Update: Also tested on preview version https://github.com/nkdAgility/azure-devops-migration-tools/releases/tag/v15.0.4-Preview.7

[MrHinsh]

Where this message is being sent its in the core field mapping. This has not applied the FieldMpas yet and will always output a warning of this type if a field on the source does not exists on the target...

[webturner]

Thanks, I re-ran with just one story in the WIQL and the target field got populated correctly.