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If the render array returned by a file includes something in its #cache property, then its possible for a file in the media browser to not be selectable, because the output was cached before it ran through the media_thumbnail wrapper.
Conversely, if you see the file in question in the media browser first after a cache clear, then the preview on the file edit page will have the media_thumbnail wrapper applied as well.
I'm seeing this with oembed that I've patched to work with the latest version of media. I did not patch the caching in, that was left untouched.