I'm sorry it's taken so long for any of us to get here.
I'm not quite sure about the time aspect of it, but my understanding is that what you have mentioned is the case - that a trade for the player was completed just before, thus invalidating the trade.
Remember, too, that the AI may have offered that player to another team in a trade several days ago, and just heard back.
At any rate, please let me know if you're seeing this a lot, or whether you'd like me to continue to research it.
Thanks,
Steve