I normally fly in OpenGL mode because I have a weather engine that will not work in Vulkan. But I also have Active Sky XP so I thought I would give it a try. I namely did this to verify if the Librain rain plugin would work in Vulkan. Sure enough it did not, but I realised something else. There was no visible rain falling. I changed over to OpenGL and restarted the last flight and sure enough I had visible rain falling. I tried several other airfields with the same result. I'm running the latest version of Active Sky XP (7542) and X Plane 11.50R3.