I have the same problem with Reason 11 full. It's a bug of the version 5 of Studio One. I think they will correct it soon.
Meanwhile this is the roundtrip:
you can change the name of the directory where reason vst is (Reason Rack Plugin.vst3):
Reason Rack Plugin.vst3
Reason Rack Plugin
The ".vst3" extension on that directory is causing the problem.
Apparently after that Reason vst is recognized and works fine.
Hope this can help you.