Design systems alone don't really solve for this. Design would need to look at the experience across the entire app.
A simplified example of how the UX can break down if frontend teams are too isolated is notifications. If every team triggers a bunch of their own notifications the user might be getting slammed with notifications.
Sure the pieces might look cohesive, but it might suck for the user if the teams aren't thinking about it from the perspective of a user.
There's always going to be shared services in any split system. Notifications is going to be one of these (kind of by necessity, especially if you plan on leveraging things like service workers which don't have a good "stacking" story to have multiple ones for the same path, so you want a shared one if you don't want every have to have one).
A simplified example of how the UX can break down if frontend teams are too isolated is notifications. If every team triggers a bunch of their own notifications the user might be getting slammed with notifications.
Sure the pieces might look cohesive, but it might suck for the user if the teams aren't thinking about it from the perspective of a user.