In Modeling mode select part of a mesh and hide the rest. Switch to Sculpt and the whole object shows until Dyntopo / Smooth Shading is selected.
After the unhidden part is shown properly, the Alt-H command has no effect to view the rest of the object. Something is interfering with this command both ways. Alt-H works IF Dyntopo is deselected and screen clicked. Then the original unhidden mesh can't be retrieved with Shift-H even while the unhidden part of the mesh is still selected in Modeling mode. To get back to the unhidden selection, go back to Modeling mode, click Shift-H as before and in Sculp mode toggle Dyntopo / Smooth Shading (on and off / or off and on). In any event I don't believe this this is working as intended.