f( x + O(x) ) or f(x) + O(x) ?

[Jacques Gélinas <jacquesg00@hotmail.com>]

In a previous post, I used this oneliner to get the vector of 
Maclaurin coefficients of a function:

maclv(f,k) = Vec( f( x+O(x^(k+1)) ) );

The boundary case k=0 should return f(0) as in
maclv( cosh, 0) == [1]
maclv( sinc,  0) == [1]

But this yields an incorrect result for this modified Riemann Xi function,

Xi(x) = my(s=1/2+x); Pi^(-s/2)*gamma(1+s/2)*(s-1)*zeta(s);
maclv(Xi,0) != maclv(Xi,1)[1]   \\ 0.67690 instead of 0.49712, *** oups!!

However, Vec( Xi(x) + O(x) ) does give the correct answer here, [0.49712].

Can I have an explanation or (better) advice regarding this behaviour of GP ?

Jacques Gélinas
Ref: http://pari.math.u-bordeaux.fr/archives/pari-users-1908/msg00022.html