When I disable lazy loading YT videos and trigger a HTML rebuild of the particular post, then the thumbnail is refreshed. But when enabling it again and rebuilding the HTML I get to see the old thumbnail again.
Is there a way to clear the tumbnail cache of one video? Or is there a way to set/check the refresh period?
This is more related to YouTube than anything else. When you disable lazy loading, the video iframe gets embeded directly which has direct access to YouTube’s API each time it get loaded.
For lazy loading, we fetch the thumbnail from img.youtube.com/vi/id/maxresdefault.jpg instead. There are multiple image sizes, but YouTube is not very clear on how they update these URLs. We use maxresdefault, which is the largest image they provide and, from my testing, is the one that usually takes the longest to update. In my case I found that thumbnails took anywhere from 1 to 30 minutes to get updated.
And to answer your last question, no, there is no way to set a refresh period. Thumbnail updates are not very frequent, so we don’t have a job that periodically updates them.
The thumbnail is updated on the YouTube side. In new topics it loads with the new one, but in the original topic it keeps reverting to the old one. As if Discourse has a cache. It even happens to this day 
My quick fix now is to disable lazy loading, rebuild the HTML of the topic, it gets the new image, but less efficient. Then I enable lazy loading again and don’t touch the topic anymore.
This is still an issue, also with new topics. Thumbnails are never updated, even if the YouTube video gets a new thumbnail. Removing the links, rebuild HTML, add the link again, all doesn’t matter. It forever keeps the old thumbnail. Can this be fixed please?
안녕하세요, 여전히 YouTube 미리보기 이미지가 표시되지 않는 같은 문제가 있습니다.
호스팅 사이트에서도 동일한 현상이 발생하고 있습니다. 유튜브 썸네일을 변경했는데 원박스가 업데이트되지 않습니다.
게시글을 편집하려고 하면 미리보기에는 새로운 썸네일이 표시되지만, 실제 게시글에는 강제로 업데이트되지 않습니다.
게시글에서 재생 버튼을 누르지 않고 원박스를 탭/클릭하면 이미지가 새로운 썸네일로 업데이트되지만, 이는 해당 게시글을 보는 동안에만 적용됩니다. 새로고침하면 다시 이전 이미지로 돌아갑니다.