Naming downloads automatically is tricky. The issue is that all sites stream in different formats and manners. Much of the time the stream being recorded comes from a completely different site than that hosting the video or music. We use a variety of techniques to try and detect the best name possible for a downloaded/recorded stream. The majority of the time Media Recorder gets it right.
However, in Settings you can switch on and off the rules Media Recorder uses to detect names.
If you are not actively trying to record anything, and oddly named files are being recorded, RMC5 might be picking up automatic updates from other applications.
In the cases where an appropriate name is not used, you can rename files directly from within the application. Just right-click the item and choose Rename from the menu.