iOS TestFlight - External Testers Not Notified of New Build

Question

I have made an app available to external TestFlight users. They have received the email and installed the app as expected. They have version 1.0, Build 1.

However, I have now uploaded a new build (Version 1.0, Build 2).

This has received approval. In the External Testing page, I have added and selected the new version. This new build now says 'Testing'.

However, the users of the previous build have not received any notification and in the TestFlight app it still says Build 1.

Does anyone know how I can make the current users get the new build? I assumed this would be automatic and I can't find any info on this situation.