When a workflow is attached to an existing content type, the default status is added to the current content items implicit. This means when the default status changes again, the document is implicitly changed again (until the document is updated and an explicit status is set).
The mean problem is this when a workflow is attached with "Published" as default status (to make sure current documents and published) and the default status is set to "Unpublished" afterwards, all unchanged documents get unpublished.
To avoid this all documents without a status should get an explicit status, but only when the Disabled status of the content item matches the Published status of the default workflow status. The matches this newly added documentation: https://www.integrated.nl/manual/workflow-instellen
In the future it would be better to allow to set new document statuses when changing a workflow, but this is currently a too big investment in the project.
Explicitly set the workflow status when the content item doesn't have a status and the Disabled status of the content item matches the Published status of the default workflow status