Workflowed Images not rendered in a Mosaic "Existing Content" tile


In one of the sites that I help administer, images are work-flowed (these are veterinary medical images).

I have recently introduced the client to Mosaic for composite pages, but we have discovered that when we use the "existing content" tile to add an image to a Mosiac layout, it is not rendered as a preview in the editor, although they show up in the page as expected. After a bit of digging it turns out that these particular images are still "private", and that "published images" behave as expected.

I'm not sure if this is a bug, or if I am doing something daft - has anyone else had to deal with this issue?