I believe the way Stream (Classic) worked with MS Forms on a video was that it only prompted you to take the quiz the first time you played the video and encountered the quiz. I may not be remembering correctly as it's been a long time since we built that. Either way you should be aware that Stream (classic) is being replaced with videos in SharePoint, Teams, and OneDrive and that we don't support Forms/Quizes on videos in SP. https://aka.ms/StreamMigration
So you might not want to rely on this feature. As a workaround you can make a MS Form and put the video as a prompt in a question.