Wow! I never knew this capability was there. I read the documentation and played a bit with it.
As described, I created an action to set values of columns. I then used the INPUT() function for a Status column. See image below.
Running the app, I selected a row and tapped the “Add Status” action. I received the result shown in the second image. Notice that the default value is not set but otherwise everything else seems ok. I did find out that if you tap the button AFTER a value had been set - that value does show in INPUT Form when it opens.
Now the question is what is different for you? Can you show your action where you have the INPUT() function defined?
Action with INPUT function defined
Result when tapping on “Add Status” button from a row
Clicking dropdown on pop-up Form