I’m not sure what triggers it, but since the Derleth patch I am seeing Spider Drones becoming inactive after 1 or 2 turns. Even the enemy no longer sees them as a threat. They just stop becoming active players even though the are shown as active player units.
EDIT: Just ran into this again. It seems that the drone “clones” itself. While there is only 1 drone on the map, if one tries to manual control it, there are 2 to choose from. No matter which one is selected, it can not be moved. The only thing that will work is to have it explode. When it explodes, the “clone” can now be controlled and moved.
EDIT Redux: It happened again. However, this time the drone wasn’t cloned, it just stopped activity. When my tech attempted to control it, there were no options (movement or explode), it was just a dud.