Black preview when uploading downloaded videos to WhatsApp

[Muatex]

bug description

When downloading videos from Cobalt.tools (tested with YouTube and X), uploading the saved video to WhatsApp results in a black preview as the video cover.
• During upload, the preview appears correctly.
• After upload completes, the preview turns completely black, even though the video is fine, and the starting frame is not black.

This issue is consistently reproducible on iOS (tested with Safari on version 18.3).

reproduction steps

1. Go to Cobalt.tools.
   2. Download a video from YouTube (e.g., save to Photos).
   3. Upload the downloaded video to WhatsApp.
   4. Notice the preview turns black after the upload finishes.

screenshots

links

platform information

Safari on iOS 18.3 iPhone 16 Pro

additional context

This issue has been observed mainly with YouTube videos downloaded via Cobalt.tools. I have also tested with X, and the same issue occurs. I’m not sure if it affects other platforms as I primarily use YouTube downloads.

[dumbmoron]

does remuxing help with this? https://cobalt.tools/remux

[Muatex]

Just tried it, it does not.