Just try doing a save of DB table instance for the Index- class created as part of your Declare index rule.
Also try saving the class definition once. and try to run the flow once. Make sure your class is mapped to DB properly and test connection is showing proper result. If still the issue is reproducible try out below stuff
Enable the declarative index rule option is tracer setting and run the flow. Please check if upon save and commit of the page with page list property, Declare index is getting fired with proper values, and not the class of the top level page is correct.