I think if default (none) clause is not used then it can be used as default private. Here in your code
- default private
do concurrent (j = 1:Ny) shared(image) local(i, j, x, y, x_0, y_0, x_sqr, y_sqr, n)
it is private by default. But in the second case it should be explicitly declared in the local
- default none
do concurrent (j = 1:Ny) default (none) shared(image) local(i, j, x, y, x_0, y_0, x_sqr, y_sqr, n)