Do not confuse visibility with reusability
An M3U8 link can look visible because a page minted it for your current session, not because it is a durable public stream. If it collapses outside that context, it was never broadly public.
Check the full HLS chain, not only the top manifest
A top manifest may still be readable while the child playlist, key file, or segment URLs use a different signature or shorter expiry. That is one of the oldest traps in media debugging.
Look for access hints in the URL and behavior
Parameters such as token, signature, auth_key, expires, or long query strings are obvious reasons to stop calling the link public without further testing. So are links that work only right after refresh or only on the source page.
Test the link after context changes
Retry after a short delay, in another browser context, or outside the original page. If the stream survives those changes while child resources also remain readable, the link is much closer to genuinely public.
Public for playback is not always public for export
Some links remain playable in the browser but still fail in FFmpeg because the request context changes. Public-for-browser and public-for-export are not automatically the same promise.
How to use m3u8play.net here
Use the player to verify whether the manifest is still readable, whether playback survives, and whether the failure appears immediately or only after deeper requests. That layered result is much stronger than a superficial tab-open test.