I’ve used Monitor to check behind the scenes. Dropdown options are coming from a SharePoint list.

Form:
Form4_1
Gallery:
Gallery1_1
Dropdown:
Dropdown11
Datacard Default:
Gallery1_1.Selected.InputID
Dropdown Default:
ThisItem.InputID
Datacard Update:
Dropdown11.Selected.InputID

When selecting an item in the gallery, the dropdown correctly sets its default value. When selecting a different value in the dropdown, it doesn’t commit to the SharePoint list in the Update.

In Monitor, I can see the patch DataOperation showing both the original and changed values. If I test using Notify(Dropdown11.Selected.InputID) in the OnSelect of the submit button, it correctly shows the selected value.

I have other forms and dropdowns working just fine using the same pattern.