With the correct video codecs installed (for the particular test video I was using, I needed gstreamer1.0-plugins-bad which pulls in libav), this now works on the mantic final images, under both the Pi 4 and 5.
With the correct video codecs installed (for the particular test video I was using, I needed gstreamer1. 0-plugins- bad which pulls in libav), this now works on the mantic final images, under both the Pi 4 and 5.