Hi! it seems I have duplicated my underwater barn, once accidentally and once in purpose.
Steps:
Go to Slamet’s Ranch, asked to edit farm building.
Move underwater barn to new location.
ask again to edit farm building, cancel movement of the barn.
Now I returned to my underwater farm and the location where I had moved the barn is marked with a fence which does not disappear the following days. Exited the game, open the game, I have a new underwater barn 🤷🏻♀️.