It was empty when I ran your macro on my High Sierra system, too. But once I ran it, it was populated. I got the same results as JMichaelTX in a notification.
I also ran a JSON lint test on your JSON and didn't find an issue. So it looks like it's fine (if confusing with the empty variable before run).
Keyboard Maestro does not simulate the entire macro to produce results for each action in the display of actions. Each action just displays the results as they currently are.
In your first image:
The current value of the “JSON” variable is empty (as shown at (1)).
The current value of the “subscriberCount” variable is empty (as shown at (2)).
Because of this, if you “Try” the last action, it will show nothing useful.
If you select all three actions and “Try”, it will set the variables and then should display the results.
Then the left hand side of the arrow (→) will show that the variable now has a value.