Thank you for your post.
Go to Field will find the first instance of that field on the layout. If you need to go to a specific instance of that field, then provide a name in the Inspector for each instance of that field and use Go to Object. In your instance, assign the name "C1" for field C in the first portal, and "C2" for the field C in the second portal. Then, change "Go to Field" to "Go to Object" and select "C2".
See the Notes section in the Help for Go to Field:
Actual this is not true. If the portal, that contains the button, also contains the field A, then the Go to Field will go to field A inside the same portal. This will happen with any similar portal on the same layout.
Now add a popover to each portal and move the field A from the portal inside the popover. Layout Objects shows, field A is inside a popover, that is inside a popover button, that is inside a button bar, that is inside a portal.
Click the button with the script again, and NOW Go to Field will always jump to first instance of that field on the layout.
Conclusion: There is a difference in the behavior, depending on the location of the field, either directly positioned in the portal or inside a popover in the portal.