If you purchase one of the Dryad's spheres, you get the popup window and correctly get it. However, the list of other spheres reloads as you do that, and it appears to only show spheres for the base 4 heroes. If you change the sphere size to whatever and then back to large, the other dryad spheres will show up then.
This may happen with other heroes, but I have all their spheres so can't check.
I have a video if you need it, and will upload later.