It is my understanding that expiring SPY option stop trading at 4pm est, not 4:15pm. Other SPY options continue to trade until 4:15pm. In fact when option stop trading, the last market at the EOD time, 4pm, is frozen and can't change or be traded electronically. I have not looked at the time and sales from friday or the current SPY or option market at that time. What is most likely, if your account of the EOD is accurate, is that at 3:59:59, SPY was trading above 209. Then after or at 4:00:00pm, the SPY had a closing print on the primary exchange lower than 209 from an imbalance. This would cause the situation you described.
Bob