Mirroring partner timeout - Not working the way advertised?
I'd suggest that the clean shutdown of the principal is causing it.
From Automatic Failover

How Automatic Failover Works

If the principal server is still running, it changes the state of the principal database to DISCONNECTED and disconnects all clients from the principal database.


One may assume that the principal says &quot;I'm DISCONNECTED&quot; on clean shutdown
To test this correctly, remove the network cables (or power leads) from the node hosting the principal to simulate/invoke a &quot;real&quot; server failure
I assume that the mirror is actually remote and not on the same cluster...