Starting with version 3.10, FTS-REST expects the params::job_metadata API field to be a JSON object.
However, the documentation on this is mixed, allowing both string and JSON format. 
Since version 3.10 appends the auth_method to the job metadata, internally, it should be treated as JSON. In case a string is received, it should be converted into a JSON object, attributing the received string to the label key.
The following submission currently fails.
However, sending a JSON object for params::job_metadata will work.