Question: How to Take Partial Derivative of a Mapping?

Here's how to break Maple's 'D' function:

~$ cmaple mapleBug.mpl
    |\^/|     Maple 2018 (X86 64 WINDOWS)
._|\|   |/|_. Copyright (c) Maplesoft, a division of Waterloo Maple Inc. 2018
 \  MAPLE  /  All rights reserved. Maple is a trademark of
 <____ ____>  Waterloo Maple Inc.
      |       Type ? for help.
> D[1]((x, y) ->x^2-y^2);
                                 (x, y) -> 2 x

> D[2]((x, y) ->x^2-y^2);
                                 (x, y) -> -2 y

> with(VectorCalculus):
> assume(x, 'real');
> assume(y, 'real');
> D[1]((x, y) ->x^2-y^2);
                                  (x, y) -> 0

> D[2]((x, y) ->x^2-y^2);
                                  (x, y) -> 0

> quit
memory used=2.4MB, alloc=8.3MB, time=0.06

~$

What if a person needs to have these lines:

> with(VectorCalculus):
> assume(x, 'real');
> assume(y, 'real');

and then he wants to take partial derivatives? Not possible? Once you load VectorCalculus and declare x and y real, Maple stops doing partial derivatives? Why.