OK I have had huge issues with uMatrix abritrarily blocking certain elements in some sites, even if I add all the exceptions or even turn off the filtering for the entire site. I have finally figured out what is going on. If anyone else encounters this, open the uMatrix Dashboard, find the option "Spoof HTTP referrer string of third-party requests" at the bottom of the main "Settings" tab, and uncheck it. So far, I have found that doing this has fixed all of the issues I had previously encountered.