I tried the blog from the excel data source for ParentReason, ChildReason. Worked fine. I must be missing something visual or otherwise on the link you provided with the Parent.Default and Cards. Writing technical steps is super-hard sometimes without screen-shots and I know I'm missing the steps.
Similar to the first example, here is what I attempted to do.
Created first drop-down from my "Contacts" data source, a SharePoint List and here's the code I placed in. I also have a second data source name "Company-Agency", which is a lookup from "Contacts" for the "Company-Agency" this individual works for.
Drop-Down Name - ContactsDrop (name matters in second drop-down)
Code - Distinct(Contacts,Name_x002d_Combined)
That worked fine. My confusion would be for the second drop-down (although I prefer a text field, just because I simply want to display the supporting fields within this Sharepoint list such as address, city, state, etc.).
Drop-Down Name - PopulatedDrop (Name shouldn't matter here)
Code - Filter('Contacts'.Company,ContactsDrop.Selected.Value in Company-Agency)