Why is Maleic Acid more soluble in water than its trans isomer Fumaric Acid?
Determining the crystal lattice energy would be helpful in comparing the starting points for a solubility investigation. Perhaps an approximation to the crystal stability is provided by the melting points: for fumaric acid, the mp is 287$^o$C; for maleic acid, 135$^o$C. 135$^o$C is a sort of normal mp for organic compounds. 287$^o$ seems quite high, suggesting a high lattice energy, and therefore, lower solubility for this isomer.

Even if molecules have the possibility of intramolecular bonding, as indicated by molecular models or drawings on paper, the existence of intermolecular bonding is not precluded.
Sometimes explanations of an observation are used as predictions which then turn out to be incorrect. The explanations may still be useful, but as explanations of actual data, rather than as reliable predictions.