I've had similar issues, but with the video playback stopping in Safari, and working in Chrome at very specific points in a few movies. When the next macOS released, things started working in Safari again.
I think Amazon/Prime Video uses an early-beta of macOS for testing and they encode their video in the most space-saving codec they can. They apparently don't care that the codec they're using is not yet available for general consumption. Be sure to click the report feedback button and complain about their broken encodings, though I doubt they'll care unless there's a sudden mass exodus from Prime Video.