Trying to understand difference between 3D and 2D plot

General Discussion
1

Dear Community,

I am trying to understand why in my 3D plot it looks like this function has a pole in the middle (centred along the line (0.5, 0,5 s)), but when I project it onto a 2D cross-section I no longer see this.

load "msh3" // to use cube

int n = 11;
real gridh = 1./n;

int c = 2;
real H = gridh*c;
cout << "support is fixed at H = " << H << ", grid size is h = " << gridh << endl;

func f = (12./(pi*H^2))*(sqrt((x - 0.5)^2 + (y - 0.5)^2)/H);
mesh3 Th = cube(n, n, n); // labels 1: y = 0, 2: x = 1, 3: y = 1; 4: x = 0; 5: z = 0; 6: z = 1;
mesh Th2 = square(n, n);

// // fespace for solution
fespace Vh(Th, P1);
Vh fh = f;

fespace Vh2(Th2, P1);
Vh2 fcut;

plot(fh, wait = true, fill = true, value = true);

// take some cuts
real[int] zcutVec = [0, 0.4, 0.6, 1];
for(int j = 0; j< zcutVec.n; j++){
  fcut = fh(x,y,zcutVec(j));
   plot(fcut, nbiso = 30, wait = true, fill = true, value = true);
}

cout << "END OF PROGRAM" << endl;