When I run the official tag-value example file through the online converter tool to JSON, it turns the tag "DocumentNamespace" into the JSON field "documentNamespace". Which makes a lot of sense, except that field does not exist in the JSON schema. Instead it's believed that the field "spdxDocument" is the intended field for the namespace. See this issue in SPDX-specs.

Personally I think it would be clearest if the schema was changed to match what the tool is putting out in this instance, but either way this is a discrepancy.