Fix 7578

[Aaronontheweb]

Fixes #7578

supersedes #7579

Changes

Please provide a brief description of the changes here.

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• This change follows the Akka.NET API Compatibility Guidelines.
• This change follows the Akka.NET Wire Compatibility Guidelines.
• I have reviewed my own pull request.
• Design discussion issue Cancelled sinks are blocking other BroadcastHub consumers #7578 and Fix #7578, Cancelled sinks are blocking other BroadcastHub consumers #7579
• Changes in public API reviewed, if any.