What problem does this address?

When applying a duotone filter to a Site Logo or Post Featured Image placeholder, there's no visual indication that anything has been applied. This was found during the 6.0 walkthrough.

What is your proposed solution?

This was previously discussed here #36517 (comment) but it doesn't seem this was ever implemented and/or a regression might have happened. cc @ajlende @jasmussen for more context.

In that discussion, it was noted that a good alternative would be to apply it to the outline and upload button as well (this button only shows when one is adding a post featured image block to a Post or Page rather than at the template level):

It could also just be added to the dashed border:

Note that the above placeholder was updated #36712 so I'm pulling in old screenshots :)