I was experiencing the same issue where I could not get the custom supports to appear in "preview", but I think that I may have discovered where the bug lies. My workaround was to temporarily turn on supports and make sure that the "Support Placement" drop-down was set to "Everywhere" and then I turned off supports. After that I was able to create my custom supports and see them in "Preview". I should mention that before this I was only able to see custom supports that actually touched the build plate because that was the setting I had in the dropdown.