Have a look at your throttle in the x-plane > settings > joystick, make sure that the lever that controls the mixture actually has 'mixture' selected in the dropdown for what it does. My levers were labelled, which was enough to get them to work on everything else I tried, but not specifically assigned to anything.which this plane needs.
I don't know what product this is, but if this is under Vulkan then it probably works by just modifying X-Plane's built-in clouds rather than replacing them entirely. That would result in reflections still working.
In contrast, SkyMaxx Pro disables X-Plane's built in clouds completely and renders its own in 3D in their place. If you're doing your own 3D drawing in a plugin, you don't have access to the reflection pass under Vulkan.