For the post object field,
render_field() only include what has been already selected. This is a select2 field and population of the rest of the field depends on an AJAX request to get the values.
To see PHP errors that happen during the request you need to turn on error logging https://codex.wordpress.org/WP_DEBUG. Basically, anything that creates unexpected output during the AJAX request will cause the request to fail and this could be why the choices are not being populated.
On the other hand it could be a JS error before the request that prevents it from happening.
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