Let me guess. You downloaded a HLS streamed video and watch it with QuickTime on your Mac, right ?
There are 2 workarounds:
- use another video player, like VLC instead of QuickTime
- convert the video (or use "Download & Convert" when requesting the download) to output format "Re-encoded MP4 (h264/aac)
For more details, please search for "QuickTime" in this forum.