Copper sulfate is also used to kill roots of trees. Perhaps you inadvertently damaged the roots of your daylilies? But how that relates to shorter scapes I cannot think.
Have you tried to purchase a new daylily as a control, to grow with the others, to determine whether it is actually the soil or your prior treatment of the daylilies?
Similarly, you can dig up one of your stunted daylilies, wash off that soil and replant in a planter to see if it recovers with different soil from elsewhere. Did you try that?
The only other thing I can think of is perhaps a shrub somewhere grew too big and is now shading the daylilies. Or that the clumps may need divided.
