I noticed a few differences between the Media library dialog when launched in Gutenberg vs when it's just on a regular Drupal field. Turns out this is because my admin theme was overriding the media library templates, which wasn't happening when the library is loaded via ajax from a non-admin route.
This is only an issue when the user is using an Admin theme, and is has the "Use the administration theme when editing or creating content" setting enabled, but I feel like that's most people these days. We can conditionally have the route be set as admin by using the _node_operation_route option that the node module uses for the same thing. MR incoming!
Fixed
3.0
Code
Automatically closed - issue fixed for 2 weeks with no activity.