Hi Phillip, thanks for asking your question! The TargetUsers associated with your WorkflowUserTask should not be empty indeed, but rather than a bug it is likely that the user task is not configured correctly. Please read through the how-to that has an example on how to link specific users to certain tasks here. I’m guessing that there’s an issue with your User Assignment property on the user task in your workflow.
If that doesn’t help you, please share a screenshot of the properties of your user task so I can help your further.