Data Integration with File - strange issue with mapping Account dimension source to targets

Summary:

I've created an integration which is driven from a file upload. I can map any dimension besides Account. No matter what criteria I enter for mapping for Account, the result in workbench shows the Account column as blank with no error (eg. no data maps to fix).

Am I doing something wrong?

It's strange that it's ONLY the column that contains the account codes which have an issue being mapped. It's as if it can't be read?!

The file is fine as I've used it in another environment where the column containing the account codes are mapped to the Account dimension in this instance with no issue.