This happens to me regularly on PVE-C official PS4 server at the Mounds of the Dead. I’ve always assumed it was intended to prevent people from walling off the obelisk.
When trying to place a foundation in the water in the north east corner I will get an invalid placement message yet it consumes the mat anyways. And I keep doing it thinking thinking it will finally accept it, to no avail.
To be clear, this isn’t a land claim issue. Lord knows I have plenty of those issues as well, however the mat is not consumed when trying to place within the land claim.
I just assumed it’s an intended mechanic in that spot but I’m not bothered by it. I just wanted to guarantee access to the obelisk. I assume my neighbor has the same issue as well so it doesn’t seem he can close off that area and leave me without quick travel options.