Mathcad is right in its answer and then its not. The correct result of your first eval should be "no solution found".
The reason is, that your problem is an integral equation with A as a function in x and y (not A but A(x,y)) but the way you wrote is denotes that A would be a constant and so Mathcad treats it as such. The solution 3xy/2, which is obviously not a constant, means that there is no solution to the problem (no constant A).
Mathcad is not capable of solving differential equations symbolically nor will it solve integral equations at all.