Personally I believe your broker may be wrong (whoever you spoke to) because there are very advanced algos that do those options assignments, while:
https://www.optionseducation.org/referencelibrary/faq/options-exercise
“Options exchanges have a cut-off time of 4:30 p.m. CT, for receiving an exercise notice. ”
A broker may show a time when they processed the assignment, but the instructions come from options clearing authority/exchange that received a request from someone else to exercise their options the night before.
And really you’ve sold a call for more money because the call price was inflated due to that dividend, and professionals don’t lose money by buying overpriced options from you
Unless your broker is so clueless that they will lose money themselves on this deal...
Also, options cannot be exercised/assigned after midnight for the simple reason of price misalignment. The exercise time would not match the stock trades and option prices that do not even trade outside regular hours. People would wait to see if they may lose money due to some after hours news and then decide to exercise their options or not.
There would be so much room for cheating it would be an absolute mess. SEC has variety of rules and regulations to prevent cheating, and some of them set specific time and conditions for exercising and assigning options.
