this is a problem i found in illustrator, indesign and photoshop cc.
i create a gradient, say from 100% to 0% of cyan (c100 m0 y0 k0). (the color is not important)
than i create another gradient, from 100% to 0% of cyan (c100 m0 y0 k0), but spot color.
than i active 'overprint preview'.
on screen, the 2 color look different, and it's wrong.
we have the same color, the same gradient, so it MUST look the same.