Did you apply Convection to the Symmetry faces? There should not be Convection on those faces.

I don't know if the Composite Cure Simulation code has been written to respect Symmetry. You have to check that.

The dialog box is only a Warning, which you can ignore since the solution did converge.

You should rerun the simulation with 1/2 size elements, and then with 1/4 size elements to see if the results change a little. Maybe the non-uniformity is evened out by using smaller elements.