After content has been published, changing alias in future draft doesn't persist after save.

In short, it appears that once you have set the path alias for a piece of content that is controlled with Workbench, it can never be changed again.

Steps to duplicate:

1. Save content and publish it.
2. Start a new draft of the content, and change the path alias for that draft. Save it.
3. Go to edit the draft. Notice that the alias has reverted back to what it was prior.