Why is magnesium ascorbate, which is a conjugate base of a weak acid, acidic?
$\mathrm{p}K_\mathrm{a}$ relation in the answer of Kav

Wikipedia says pKa's are 4.10 (first), 11.6 (second)

gives the hint a monobasic salt with alkali metals should be very mildly alkalic, as pKa2 is farther from pH 7.
$pH \approx \frac{\mathrm{p}K_\mathrm{a1} + \mathrm{p}K_\mathrm{a2}}{2} = \frac{4.10+11.6}{2}=7.85$
OTOH, it will probably form a complex with Mg, affecting acidity of the 2nd hydrogen. Like
$$\ce{2 H2A(aq) + Mg^2+(aq) -&gt; [MgA2]^2-(aq) + 4 H+(aq)}$$
respectively
$$\ce{Mg(HA)2(s) -&gt;[H2O] [MgA2]^2-(aq) + 2 H+(aq)}$$
See also citrate or EDTA complexes for this effect. AFAIK, before suitable metalochrome indicators came, metal content with excess of Na2H2-EDTA had been sometimes determined by back titration using NaOH.