I have the same problem.
In surf or mesh, datatip with coordinate x,y,z may not work correctly.
datatip with 'DataIndex' may work correctly.
For DataIndex, sub2ind can be used.
In this case, below code may work correctly.
a = 0 : 0.1 : 1;
b = 0 : 0.1 : 1;
[bb,aa] = meshgrid(b,a);
Acc = aa.^2 - bb.^2;
ss = surf(aa,bb,Acc);
c = a(2)^2 - b(4).^2;
id=sub2ind(size(Acc),2,4);
datatip(ss,'DataIndex',id);