I have seen that same thing from several websites. I have seen some websites suggest it's a Safari issue (browser hijackers) and clearing Safari data gets rid of it. But, I have not really messed with it yet.
It takes getting used to but really allows you granular control of 3rd party requests on both a site-by-site and global basis.
Word of warning, most uMatrix users I've met say it becomes depressing to see how many 3rd party requests most websites require before they are even usable. It becomes very obvious very quickly.