NDSolveValue not working correctly

So I am trying to get a good representation of the 2d heat equation with mathematica but it does not seem to work correctly:

heqn = D[u[t,x,y], t] -Laplacian[u[t,x,y], {x,y}]

shape = Rectangle[{0,0},{10,10}]

ic = u[0,x,y] == x+y

bc = NeumannValue[0, True]

sol = NDSolveValue[{heqn == bc, ic}, u, {t, 0, 300}, {x,y}[Element]shape]

Manipulate[Plot3D[sol[t,x,y], {x, 0,10},{y,0,10},ColorFunction->Function[{x,y,z},Hue[z]]], {t,0,300}]

Initial Condition :

The expected result is equilibrium after some time but instead the heat is just transferring over and causing really weird things to happen in the plot: