I’ve done some debugging and found that WP is preventing the embed preview.
There is some code within the WP preview function that prevents the embed form being loaded / returned (during ajax call) if the post being edited does not exist.
The post will never exist because you are editing a term, not a post.
This security measure is stopping the preview from working, however, the embed should still work when outputting the value on the template. Can you test this?
I’ve added this to my to-do and may be able to trick WP into thinking it is editing a post instead of a term!
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