To be fair, the idea that Donald Trump could hold a conversation with someone and walk away not knowing who that person was or not remembering that person's role in the conversation doesn't seem implausible. He is both narcissistic and delusional. There's no particular reason to believe that he has actual awareness of who he is talking to at all times.
And if he just happens to demand firing an ambassador during that conversation, for most people that would be a pretty big deal and strong evidence that they were actually engaged in that conversation as a serious matter, but for Trump it could just be an impulsive outburst forgotten before dessert. He obviously didn't act on his own demand that Yovanovitch be fired at the time, so it's quite possible that he just forgot, and no one reminded him. A year later, the subject came up again and this time she was actually fired.