Currently, the graphql-code-generator when generating the Data types replaces all enums with strings. It makes sense from a factual point - the data in a JSON format (coming from a REST call or mongodb query or most other things) will contain a string, not enum. Nonetheless, that makes testing harder, and the code less strict.
We have a terrible hack for it, and plan to fix it in a plugin at some point.
For now - add lines like these to your top-level fix-generated.js .