i had the same issue. what worked for me, cut out the landscape (ctrl + x) and pasted it again into the same level.
bad side, you lose all your splines. good side, undeleteable splines are gone.
i had the same issue. what worked for me, cut out the landscape (ctrl + x) and pasted it again into the same level.
bad side, you lose all your splines. good side, undeleteable splines are gone.