Apparently, the latest update of microsoft flight simulator (188.8.131.52 - that is the "VR" release) has a problem with PartInfo "mouse drag" actions. Basically the legacy code for mouse drag action in the Virtual Cockpit does not seem to work anymore - this affects both the MB-339 and the LONG-EZ and probably affects all the aircraft that use the PartInfo method for the model animations (but not the ones from Asobo, as they use a different method).
I have already notified microsoft of the problem.
It is not a huge problem for most of the controls, as they can also be operated with the mouse wheel, but there is a major issue with the Long-EZ fuel selector which cannot be operated with the mouse anymore.
As a temporary workaround (waiting for microsoft to fix this) you can assign keyboard shortcut or a joystick/controller button to control the fuel selector. The functions you need to assign are "FUEL SELECTOR 1 LEFT" and "FUEL SELECTOR 1 RIGHT".
Apologies for the inconvenience.