That is perfectly normal. Hidden geometry is not affected by any sculpting or deformations.
After sculpting one of the hands while the other is hidden, unhide the whole model and apply a mask to the previously hidden hand . Invert the mask ( Ctrl + i ) and go to the lowest subdivision level. Now press Tool > Deformation > Smart ReSym with the relevant XYZ axis enabled. Go up a subdivision level and apply Smart ReSym again, repeat for the remaining subdivision levels.
I always have fine results applying Smart ReSym just once, at the highest subdivision level. But apparently some errors can occur if you do not do it from the lowest to the highest subdivision level.
You could of course also leave both hands visible and enable Transform > Local Transformations to make your modeling focus and rotation stay on one of the hands.