Bruh. The users are from feddit instance but the community they're posting on is world instance.
It doesn't work like that, even if the post is on another instance the media itself is hosted on the users' instance. For example if a feddit.org user posted to a dbzer0 community I wouldn't be able to see it, since the image itself is hosted on feddit.org.
There's a fix for this called image proxying, which a few instances like Lemmy.zip for example enable, but Lemmy.world doesn't do this.
And I can see the media when I open it in a browser without logging in.
Then I have no idea what's going on, sorry.