If you are working as an external contractor, it isn't done until it's signed. This isn't Thompson Reuters hiring you as a permanent employee, it's them hiring you versus someone from a bodyshop.

Without knowing their side of the story, it does seem like a poor move on their part. But.... I've seen situations where people have told firms, "You are my first choice" only to say, "I need some time to think about it" after receiving the offer. It definitely ticks off the hiring manager, who then thinks they misread the enthusiasm, and their offer is being shopped. Could something like that have happened here?