Currently, Simple Search cannot be used with Shared Dashboards. We’ve limited this currently so that there is no risk of your data (like assignees, reporters, sensitive custom fields, etc.) being exposed. Because more sensitive fields like user pickers are available in Simple Search, we do not currently allow these to be added to your Shared Dashboards.
In the future, we will be looking to allow Simple Search on your Shared Dashboards in a secure way.