YouTube & Vimeo will not play on most mobile devices. The Placeholder image will be displayed instead when it can’t.
This is not a bug, we cannot override something like this as it device-related not theme related. Some devices block auto-playing videos from services like Youtube and Vimeo.
We strongly recommend uploading MP4 video in settings > files in the admin of Shopify as they are the most likely to play on most mobile devices. Youtube videos are not MP4 files