Variable default value erases manual value when source changes

[DrKEWatson]

So I have a setup where one combo box uses the selection of another to set its default value. It is a very simple code:

If (!(DataCardValue77.Selected.Value = "No App Impacted"), First(Choices([@SourceName].Issue, "Software/Application) 

This works fine. But the problem I am facing is that when the user has manually selected a value in this combo box, and then changes the source control (DataCardValue77) it resets the control to the new default. I was under the impression that the default value only mattered if you hadn't put in a value but this seems to not be the case. Is there a work around for this so that it will only change if the combo box had no value selected?

Any help would be appreciated.

[DrKEWatson]

So I found that if I do an OnChange for the source combobox to set a variable to be either the current selected record in the second combobox or if the second combobox is empty set the variable to the default I want ("Software application") I can then use the variable to be the default value. This means that if a value is alredy present it will become the default if I change the source combo box but if no value is selected it will default it to the right value. I just have to make sure I reset the variables to an empty record after submitting or clearing the form.

But it works.