After formation member ends walking, he tries to face target point. However that eliminates rotation set by SetInPosition from formation component. Partly fixed in rP23248 with setting correct rotation in idle state, but one could still notice that there is glitch when unit does quick turn to target point and then back. However that solution would be still required if logic in SetInPosition would not be changed to always set correct rotation. Another alternative would be to unset unit at position with every walking order but that would have the same effect and this is just more performance friendly to not splice member from position every time.
Notice that inPosition is currently useless as it is and had more usages across history. (rP12624, rP12636, rP12563, but current code does not really take any advantage from that information). I am avoiding to remove it entirely as I plan to reuse that logic a bit differently.