Environment is Wordpress 4.94 Multisite with numerous blogs up and running. This issue affects this blog only and is not affecting the others.
When uploading an image or PDF (or other asset) to the Media Library the upload works and the file can be sourced on the server. Image sizes are generated as expected, and all urls to those files are accessible in the detail view in Media Library.
However when uploading recent files to the Media Library the default placeholder image is being shown, not the thumbnail for the image or the PDF icon for those files.
Additionally - the key difference to other similar issues - is that the filename is not showing on top of the thumbnail nor is it seen in the detail view.
Upon investigating the wp_postmeta table I have discovered the new uploads have no entries.
I have tried the usual bug-finding steps such as turning off individual plugins and setting the theme to twentysixteen but it persists.
Any suggestions or ideas on how to debug this further?