Well as no-one has replied Tom I'll jump in here and let you know that you are not alone although it will be no consolation I am sure.
I too have this problem - intermittently I may add.
I have taken to keeping all of my alerts in a standard Notebook .txt file so when it happens I can quickly & easily cut & paste the missing Mode-S, Registration, Aircraft & Squakws into the Alert pane in RB.
You might want to do the same if you have a lot of Alerts as it saves a lot of grief.
Hope this is fixed in the next version...
Kind regards,
-=Glyn=-