I'm not sure how this happened, but literally half of the squads wandering around have no target assigned. The amount of not occupied smarties is definitely sufficent for them to decide where they should go.
This occured after changing priority of smart terrains in simulation_objects_props.ltx. Could this be caused by too big gap between most focused targets and the rest of them? I left a free room in case some other priorities would be needed.
I'm leaving the file below, will try with other values in the meantime.