The documentation for the getWorkflowName method takes two parameters:

public String getWorkflowName (Project project, String issueType)

The second parameter is a String type named issueType, but it's not clear whether an issue type name or ID (or some other value) should be passed here. Passing the issue type name succeeds (no exception is raised), but the return value is the workflow name for unassigned issue types. This behavior is somewhat unexpected. Passing the issue type ID as a string results in the correct behavior.

Please update the documentation to reflect that this should be the issue type ID as a string that gets passed as the second parameter.

Reference:
https://developer.atlassian.com/static/javadoc/jira/latest/reference/com/atlassian/jira/workflow/WorkflowSchemeManager.html#getWorkflowName(com.atlassian.jira.project.Project, java.lang.String)