I have created a reusable workflow against a Content Type. and using SharePoint Online (O365) and developing the workflow in SharePoint Designer 2013.
I can build the workflow, save it, publish it and associate it with a list. I've even tested the workflow against a list item and worked fine. later next day came and tested the same workflow never worked.
When I reopen the workflow, the Content Type is no longer associated with the workflow, and thus to make edits, I must copy the logic into a new workflow associated with the content type.