Pond Fill Tool Not Working Properly

As a work around if you want to have a fence enclosing your back yard but still have a pond. I used the same trick people use to add tiles to tiny houses without the game counting them. You delete one section of fence, recreate that piece with the same angle somewhere where it is not connected to anything. Then using the select tool move that freestanding section of fence to the open space, completing your desired fence without the game turning it into a "room".