The Share button and dialog just sends a link and doesn't change the actual permissions of the video so that the recipients can see it. This is a legacy behavior and doesn't represent the new behavior of other Office 365 services like OneDrive, SharePoint Online, and Teams. If the owner's intent was to share the file to someone without permissions, the permissions should follow. Closely aligning Stream's Share functionality with OneDrive's behavior would be a lot easier on your end users and for them to understand.
The Permissions dialog in a video (if you can find it) needs to be made more clear. "Update video details" and "Add to group/channel" doesn't give me much clue that I am going to be managing the permissions of the video. Closely aligning Stream's Permissions functionality with OneDrive's behavior would be a lot easier on your end users and for them to understand.
... View more