Hello and welcome to our forums,
Thank you for using Advanced Installer.
This is our current implementation. I've created a ticket in our bug tracking system and so we analyze our current implementation and maybe we will consider not displaying the uninstall reason form during a silent uninstall process. I agree with you that this defeats the purpose of an unattended process.
In the meantime as a workaround, you can proceed like this:
1. go to "Custom Actions" page and add a new "Set installer property" custom action with sequence like this:
Condition: UILevel = "2" AND REMOVE = "ALL"
2. schedule the action under the "Install Execution Stage" just before "Searches" action group
3. rebuild and test the silent uninstall scenario
All the best,