The latent spoiler logic within MSFS is highly unrealistic. In order
to model the three spoiler mechnanisms (spoilerons/flight
spoilers/ground spoilers) we have added a significant amount of logic
that overrides the latent MSFS controller. Sometimes this works well,
and sometimes it does not. We are finding that it depends mostly on how
the user interfaces with the spoilers - and while we will continue to
refine this logic in order to make it behave seamlessly, we have some
"best practices" for crews in the meantime.
Arm the spoilers only in the 2D Throttle panel but using the
mouse to drag the spoiler handle to the ARM position. From this point,
you can use the "/" key to deploy and stow the spoilers if you wish.
(You can also deploy the spoilers from the DOWN position using the "."
key at any time.) Note that when you stow the spoilers using the "/"
key, they will return to their previous position. (either DOWN or ARM)