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.
Having a similar issue myself but instead of the showing only the spheres for the 4 base heroes my list appears completely blank, This might be due to me having all the spheres already?
Changing the sphere size does not fix anything and I had to leave and re enter the menu to refresh the list. This had to be done for every purchase.
Thank you for submitting this bug. This is a known issue, and we are working on a fix for it.