When embedding videos from the Microsoft Stream Video service into rich text areas like shape notes, no video thumbnail is visible and instead a generic Microsoft Stream logo is displayed.
Microsoft Stream does not support video thumbnails via OEmbed at this time.
When embedding Microsoft Stream videos on a Dashboard via the Video Gadget, a thumbnail will appear. This is because on the Dashboard, the video actually gets immediately loaded, which shows the Thumbnail within the video player. This is not an option in other rich text areas as it would cause a lot of potentially unwanted requests to the video URL when perusing the platform.