I use to ensure devtools > Networking is open when I press play (or reload the tab if it's autoplaying). If it's streaming the timeline should make it obvious which resource it's about. Then just right click on the item and "Open in new tab" or even Save (I don't remember if it's an option). When in a new tab, just save the page as the page will be the media file itself.
FYI, this level of interference is trivially circumvented using several of the possible alternative methods for serving video, even without applying actual DRM wrappers.
