Issue Summary

This is reproducible on Data Center: yes

Replicated on 9.2.0-beta2, 9.0.3, 8.5.17 and 8.5.16, could not replicate on 8.5.15. Likely a regression between 8.5.15 and 8.5.16

Steps to Reproduce

1. Attach an image to a page and publish it
2. Delete the attachment from the page's attachment menu
3. Observe "Unknown Attachment" placeholder
4. Edit the page and attach a new, different image (with different filename) (with Unknown Attachment placeholder still present)
5. Publish the page

Expected Results

Unknown Attachment placeholder still exists, and new image appears, so both placeholder and new image are present

Actual Results

Unknown attachment is replaced by the new image on view page. Going back into edit mode shows that the placeholder has been replaced by the image in the editor too. This occurs with as many occurrances of Unknown Attachment placeholders as exist on the page (Only if the unknown attachment placeholders are from missing IMAGES, missing files like PDFs are not replaced)

Loom demonstration: https://www.loom.com/share/2d3b9621d6c347ac8f3bc47c7575b6cf?sid=177d593d-5a9e-4fd2-b102-1710e48401de 

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available