Called Market Surveillance, Complaints and Inquiries they said under UMIR 10.9D (Universal Market Integrity Rule) the trade was re-adjusted. There was an issue with the trading, could have been a number of things that caused it.
When they see unusual trading, they can vary or cancel a trade which in the opinion of Market Integrity Official is deemed unreasonable.
It doesn't happen often.
I was told the reason for the drop is not made public.