It is my limited understanding that attribute rules can only use data from related database views if the view is registered with the geodatabase.
@RPGIS in https://community.esri.com/t5/developers-questions/potential-bug-regarding-views-created-in-an-sde/m...
It turns out that in order for attribute rules to identify any view(s) the view(s) need to be registered with the [geo]database.
I have many database views that I want to keep as unregistered for ease of updating the SQL. Otherwise, every time I want to make a small change to the SQL, the view would need to be deleted, recreated, re-registered, and privileges redefined. That is an inefficient process and is error-prone.
It would be a shame to move away from unregistered views, which are easy to manage, just to use them in attribute rules occasionally.
Could attribute rules be enhanced so that they can use unregistered views? Especially for non-spatial database views, which are simpler than spatial views.
Related idea: Unregister with Geodatabase GP Tool