Yes, sorry I saw that now, you use low pH for the isomerization. And your question is reasonable. Maybe the conjugation with the two carbonyls makes the double bond so stable it will not hydrate? Maybe you do have the hydrate as an intermediate and this looses water again to form the fumaric acid, again, because the double bond can conjugate? The maleic anhydride is probably so easy to hydrolyse that the isomerisation does not take place at this pH.