This is an "unintended screen" that Google is aware of, and they're working with the developer to fix the issue.
Game updates are a very common thing to see and experience on pretty much every platform around, apart from Google Stadia that is. Since these games aren't stored on any physical drive and are instead played via Google's up-to-date servers, it caused quite the confusion when Reddit user 121910 spotted NBA 2K20 on Stadia downloading an update, sharing a screenshot of the bizarre message on the Stadia subreddit.
It's unknown why or how this could happen, since the downloaded data is essentially going nowhere, and it even breaks one of the core ideals of Google Stadia, which is stated very clearly on the official website as 'no waiting for game downloads'.
Responding to the post, a Google community manager explained a little about what was going on.
"Thanks for bringing this to our attention. This is a bug within the game that's showing an unintended screen," they wrote, and apparently it won't be a simple fix either.
"We're working with the developer on this, and I'll report back once there's an update."
They later updated the comment with a bit more information, saying that "we're continuing to work diligently with the developer on this matter. We're looking to have another update for you all after the Thanksgiving holiday. In the meantime, NBA 2K will play correctly after a brief update. Thanks for your patience!".
It's been quite a hectic launch for Google Stadia, especially since there have been concerns that various games aren't hitting the promised 4K mark on the platform.