The terrain is picked from the Geoscape texture. When no texture is available, Moon is used. I specifically chose Moon to immediately notice if something is wrong with the mission generation.
But you can easily alter your relevant deployment (in this case STR_CULT_INVESTIGATION_ZSRR) and change MOON into something else, for example FOREST. It shouldn't even have any impact from the game, since default terrain is ignored (as the terrain is taken from the globe).