If the games were legacy titles from now defunct studios then it would make sense for 3rd parties to be able to update the box art, but both of those games you mentioned are new releases, from game studios that are still very much in business.
As I previously mentioned your best way forward would be to reach out to the developers and explain the issue with the lack of box art on Twitch and how they resolve it, as it’s for them to decide what artwork to use (and for all you know, perhaps their marketing team doesn’t want to use certain artwork there which may be why it’s left blank).