I don't know, having been through a number of B2B contract negotiations (in Australia). It is not at all surprising for one party to make changes without highlighting. I've even seen track changes on a word document disabled, changes made, and then reenabled so as to intentionally hide word changes.
You only have to notify people of a change, if it's already signed (and generally that's because your contract will have some form of amendment system that involves notification).