From https://muffinman.io/blog/hack-for-ios-safari-to-display-html-video-thumbnail/
By simply adding #t=0.001 at the end of the video file url, we are telling the browser to skip the first millisecond of the video. When you do this, even iOS Safari will preload and show that specific frame to the user.
So the code looks like this:
<video>
<source src="path-to-video.mp4#t=0.001" type="video/mp4" />
</video>
KDSinc
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Also, if you have preload="none" attribute in your code, it will also stop the thumbnail from showing when used with #t=0.001