I am experiencing this as well:
Here is the most recent tweet where this occurs on my iOS device:
Here are the meta tags I’m defining in the linked page:
<meta name=twitter:title content="Lemmy Koopa">
<meta name=twitter:image content=https://s3-us-west-2.amazonaws.com/media.randomvictory.com/victory/5886d552e0bdfe067840ee41/frame-00001.png>
<meta name=twitter:image:alt content="Lemmy Koopa">
<meta name=twitter:card content=player>
<meta name=twitter:url content=https://randomvictory.com/victory/5886d552e0bdfe067840ee41>
<meta name=twitter:player content=https://randomvictory.com/v/5886d552e0bdfe067840ee41>
<meta name=twitter:player:stream content=https://s3-us-west-2.amazonaws.com/media.randomvictory.com/victory/5886d552e0bdfe067840ee41/moment.mp4>
<meta name=twitter:player:stream:content_type content=video/mp4>
<meta name=twitter:player:height content=224>
<meta name=twitter:player:width content=256>
If I don’t specify the player:stream and player:stream:content_type then it defaults to the player url which works. Still native streams work better in apps that read the Twitter Card information so I would prefer to use the player:stream