I think they're replicated asynchronously, so reading directly from the replica may return old data. That's why they've added the ability to deploy special workers that "live" closer to the primary:
> Embedded compute
> But we're going further. With D1, it will be possible to define a chunk of your Worker code that runs directly next to the database, giving you total control and maximum performance — each request first hits your Worker near your users, but depending on the operation, can hand off to another Worker deployed alongside a replica or your primary D1 instance to complete its work.
> Embedded compute
> But we're going further. With D1, it will be possible to define a chunk of your Worker code that runs directly next to the database, giving you total control and maximum performance — each request first hits your Worker near your users, but depending on the operation, can hand off to another Worker deployed alongside a replica or your primary D1 instance to complete its work.