It would not move because formation controller was not told to attack but unit's did it individually.
Were they in defensive stance? In that case they would return to held position given by last player command while being still in formation they would run. However they could reform at place and then walk to held position as formation but current code about held position is rabbit hole more with formations.
Even if they would not be in defensive stance to go back to held position automatically reforming after attack/action (when members are idle again) while half of formation may be elsewhere will trigger run too.
Now reason why they run in formation, if they would stuck and need to catch up with formation and if they would not run they would never be able to.
Looking into the code I can see potential line, causing them run when they act by themselves but I need to check unitai. Previously it checked if they are tasked to move as formation, now it checks if they are member of formation. However check is now more correct so we need to fix unitai and formations what is long topic or bring back old logic and rename the function, but reforming will still trigger run.
We could disable running for formation members but they would never catchup so that's probably not good idea eighter.