This kind of requirement is normally handled a bit differently in Mendix.
A common approach is to place the data that should change after the dropdown selection inside a Data Grid or another data widget, and source that widget with a non-persistable entity. Then, give the dropdown an on change action.
In that on change nanoflow, update the non-persistable object that is used by the widget, and make sure the client is refreshed. For that, you can try:
So instead of trying to directly “pull back” in-memory imported data to the page in a custom way, the usual pattern is:
dropdown change → update NP entity → refresh client → widget re-renders with the new values.
If that still does not work, then the next things to check are whether the page widget is actually bound to the same non-persistable object being updated, and whether the nanoflow is running in the correct context.
If this resolves your issue, you can mark it as accepted.