I did exactly 5 runs. 2 on Monday, and 3 today before the end of the event period but was not credited for it. Meanwhile, I had also completed the reach 200 event on the last day as well but was credited for it.
I'll be losing out of the mount, 1k reward pts and 12 enhancement scroll as a result.
You made the post at ~4:40pmPST, 40m after 12/19->12/20 reset, hence you were 40m too late. Also, could be wrong, but my guess is that mission progress is only updated once every 8hrs, so if you finish one after the last check, it doesn't count is my guess.