From my observation
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Onebox is cached for ~24 hours.
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Onebox regenerate on rebake when cache expired.( it might depend how it got rebaked)
It mean that if a post get linked multiple times in a period of 24 hours and the post was modified. it will not reflect in the newly generated onebox.
It might also be possible that if a post is cached in onebox in the 24 hours period it will be possible to generate the onebox for a deleted post.