When I have run into this, I try to duplicate the build in a different area, just to see if it is a placement/order issue or a “can’t build here” situation. Sometimes it is an order of placement issue and putting down the tiles in a different order will work. This is akin to putting fence foundations on a foundation prior to placing stairs. The opposite will not work.
Since it is the same space, vertically, it makes me thing it is a land claim issue. Have you tried getting on the ground and trying to place a a tile toward the fish camp? The issue could very well be the foundation is actually overlapping their claim on the edge, which prevents fences added to that side but not placement of the original tile. I have had this happen to me in several areas butting against no build zones.